Add ssl_set_arc4_support()

Rationale: if people want to disable RC4 but otherwise keep the default suite
list, it was cumbersome. Also, since it uses a global array,
ssl_list_ciphersuite() is not a convenient place. So the SSL modules look like
the best place, even if it means temporarily adding one SSL setting.
diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index 01b0aca..d611920 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -847,6 +847,10 @@
         suite_info->max_minor_ver < ssl->minor_ver )
         return( 0 );
 
+    if( ssl->arc4_disabled == SSL_ARC4_DISABLED &&
+            suite_info->cipher == POLARSSL_CIPHER_ARC4_128 )
+        return( 0 );
+
 #if defined(POLARSSL_ECDH_C) || defined(POLARSSL_ECDSA_C)
     if( ssl_ciphersuite_uses_ec( suite_info ) &&
         ( ssl->handshake->curves == NULL ||