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_cli.c b/library/ssl_cli.c
index be80de7..0c587e9 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -1762,12 +1762,7 @@
 
 #if defined(MBEDTLS_ZLIB_SUPPORT)
     /* See comments in ssl_write_client_hello() */
-#if defined(MBEDTLS_SSL_PROTO_DTLS)
-    if( ssl->conf->transport == MBEDTLS_SSL_TRANSPORT_DATAGRAM )
-        accept_comp = 0;
-    else
-#endif
-        accept_comp = 1;
+    accept_comp = MBEDTLS_SSL_TRANSPORT_IS_TLS( ssl->conf->transport );
 
     if( comp != MBEDTLS_SSL_COMPRESS_NULL &&
         ( comp != MBEDTLS_SSL_COMPRESS_DEFLATE || accept_comp == 0 ) )