Use new tools for all cases with TLS-specific code

This commit handles occurrences of case 2 and 3 in the following list:

1. Some DTLS-specific code with no TLS-specific code (most frequent)
2. Some specific code for each protocol
3. Some TLS-specific code with no DTLS-specific code (least frequent)

Case 3 previously had a weird structure in that the TLS-specific code was
always present, but the if structure was conditional on DTLS being enabled.
This is changed by this commit to a more logical structure where both the code
and the test are conditional on TLS being enabled.

Case 2 doesn't require any change in the code structure in general. However,
there is one occurrence where the if/else structure is simplified to assigning
the result of a boolean operation, and one occurrence where I also noticed a
useless use of `ssl_ep_len()` in a TLS-specific branch, that I turned to the
constant 0 as it makes more sense.

Case 1 will be handled in the next commit, as it can easily be handled in an
automated way - only cases 2 and 3 (sometimes) required manual intervention.

The list of occurrences for cases 2 and 3 was established manually by looking
for occurrences of '= MBEDTLS_SSL_TRANSPORT_' in the code and manually
checking if there was a TLS-specific branch.

New sizes (see previous commit for the measuring script):

```

both
   text    data     bss     dec     hex filename
   1820       0       4    1824     720 debug.o (ex library/libmbedtls.a)
      0       0       0       0       0 net_sockets.o (ex library/libmbedtls.a)
    548       0       0     548     224 ssl_cache.o (ex library/libmbedtls.a)
  11155       0     596   11751    2de7 ssl_ciphersuites.o (ex library/libmbedtls.a)
  17156       0       0   17156    4304 ssl_cli.o (ex library/libmbedtls.a)
    460       0       0     460     1cc ssl_cookie.o (ex library/libmbedtls.a)
  17649       0       0   17649    44f1 ssl_srv.o (ex library/libmbedtls.a)
    800       0       0     800     320 ssl_ticket.o (ex library/libmbedtls.a)
  39286      60       0   39346    99b2 ssl_tls.o (ex library/libmbedtls.a)
  88874      60     600   89534   15dbe (TOTALS)

DTLS-only
   text    data     bss     dec     hex filename
   1820       0       4    1824     720 debug.o (ex library/libmbedtls.a)
      0       0       0       0       0 net_sockets.o (ex library/libmbedtls.a)
    548       0       0     548     224 ssl_cache.o (ex library/libmbedtls.a)
  11155       0     596   11751    2de7 ssl_ciphersuites.o (ex library/libmbedtls.a)
  17068       0       0   17068    42ac ssl_cli.o (ex library/libmbedtls.a)
    460       0       0     460     1cc ssl_cookie.o (ex library/libmbedtls.a)
  17553       0       0   17553    4491 ssl_srv.o (ex library/libmbedtls.a)
    800       0       0     800     320 ssl_ticket.o (ex library/libmbedtls.a)
  38499      60       0   38559    969f ssl_tls.o (ex library/libmbedtls.a)
  87903      60     600   88563   159f3 (TOTALS)

TLS-only
   text    data     bss     dec     hex filename
   1820       0       4    1824     720 debug.o (ex library/libmbedtls.a)
      0       0       0       0       0 net_sockets.o (ex library/libmbedtls.a)
    548       0       0     548     224 ssl_cache.o (ex library/libmbedtls.a)
  11155       0     596   11751    2de7 ssl_ciphersuites.o (ex library/libmbedtls.a)
  14912       0       0   14912    3a40 ssl_cli.o (ex library/libmbedtls.a)
    460       0       0     460     1cc ssl_cookie.o (ex library/libmbedtls.a)
  15868       0       0   15868    3dfc ssl_srv.o (ex library/libmbedtls.a)
    800       0       0     800     320 ssl_ticket.o (ex library/libmbedtls.a)
  27619      60       0   27679    6c1f ssl_tls.o (ex library/libmbedtls.a)
  73182      60     600   73842   12072 (TOTALS)
```
diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index c152bc3..3704f6a 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -1304,12 +1304,13 @@
 
     buf = ssl->in_hdr;
 
-#if defined(MBEDTLS_SSL_SRV_SUPPORT_SSLV2_CLIENT_HELLO)
-#if defined(MBEDTLS_SSL_PROTO_DTLS)
-    if( ssl->conf->transport == MBEDTLS_SSL_TRANSPORT_STREAM )
-#endif
-        if( ( buf[0] & 0x80 ) != 0 )
-            return( ssl_parse_client_hello_v2( ssl ) );
+#if defined(MBEDTLS_SSL_SRV_SUPPORT_SSLV2_CLIENT_HELLO) && \
+    defined(MBEDTLS_SSL_PROTO_TLS)
+    if( MBEDTLS_SSL_TRANSPORT_IS_TLS( ssl->conf->transport ) &&
+        ( buf[0] & 0x80 ) != 0 )
+    {
+        return( ssl_parse_client_hello_v2( ssl ) );
+    }
 #endif
 
     MBEDTLS_SSL_DEBUG_BUF( 4, "record header", buf, mbedtls_ssl_in_hdr_len( ssl ) );
@@ -1407,13 +1408,19 @@
             return( ret );
         }
 
-    /* Done reading this record, get ready for the next one */
+        /* Done reading this record, get ready for the next one */
 #if defined(MBEDTLS_SSL_PROTO_DTLS)
-        if( ssl->conf->transport == MBEDTLS_SSL_TRANSPORT_DATAGRAM )
+        if( MBEDTLS_SSL_TRANSPORT_IS_DTLS( ssl->conf->transport ) )
+        {
             ssl->next_record_offset = msg_len + mbedtls_ssl_in_hdr_len( ssl );
-        else
+        }
+        MBEDTLS_SSL_TRANSPORT_ELSE
 #endif
+#if defined(MBEDTLS_SSL_PROTO_TLS)
+        {
             ssl->in_left = 0;
+        }
+#endif
     }
 
     buf = ssl->in_msg;
@@ -1595,7 +1602,7 @@
      * Check the cookie length and content
      */
 #if defined(MBEDTLS_SSL_PROTO_DTLS)
-    if( ssl->conf->transport == MBEDTLS_SSL_TRANSPORT_DATAGRAM )
+    if( MBEDTLS_SSL_TRANSPORT_IS_DTLS( ssl->conf->transport ) )
     {
         cookie_offset = 35 + sess_len;
         cookie_len = buf[cookie_offset];
@@ -1650,9 +1657,13 @@
      */
         ciph_offset = cookie_offset + 1 + cookie_len;
     }
-    else
+    MBEDTLS_SSL_TRANSPORT_ELSE
 #endif /* MBEDTLS_SSL_PROTO_DTLS */
+#if defined(MBEDTLS_SSL_PROTO_TLS)
+    {
         ciph_offset = 35 + sess_len;
+    }
+#endif
 
     ciph_len = ( buf[ciph_offset + 0] << 8 )
              | ( buf[ciph_offset + 1]      );