darwinssl: Warn that disabling host verify also disables SNI

In DarwinSSL the SSLSetPeerDomainName function is used to enable both
sending SNI and verifying the host. When host verification is disabled
the function cannot be called, therefore SNI is disabled as well.

Closes https://github.com/curl/curl/pull/1240

docs/libcurl/opts/CURLOPT_SSL_VERIFYHOST.3

@@ -58,9 +58,16 @@ The default value for this option is 2.
 
 This option controls checking the server's certificate's claimed identity.
 The server could be lying.  To control lying, see
-\fICURLOPT_SSL_VERIFYPEER(3)\fP.  If libcurl is built against NSS and
-\fICURLOPT_SSL_VERIFYPEER(3)\fP is zero, \fICURLOPT_SSL_VERIFYHOST(3)\fP is
-also set to zero and cannot be overridden.
+\fICURLOPT_SSL_VERIFYPEER(3)\fP.
+.SH LIMITATIONS
+DarwinSSL: If \fIverify\fP value is 0, then SNI is also disabled. SNI is a TLS
+extension that sends the hostname to the server. The server may use that
+information to do such things as sending back a specific certificate for the
+hostname, or forwarding the request to a specific origin server. Some hostnames
+may be inaccessible if SNI is not sent.
+
+NSS: If \fICURLOPT_SSL_VERIFYPEER(3)\fP is zero,
+\fICURLOPT_SSL_VERIFYHOST(3)\fP is also set to zero and cannot be overridden.
 .SH DEFAULT
 2
 .SH PROTOCOLS

lib/vtls/darwinssl.c

@@ -1425,6 +1425,9 @@ static CURLcode darwinssl_connect_step1(struct connectdata *conn,
             "the OS.\n");
     }
   }
+  else {
+    infof(data, "WARNING: disabling hostname validation also disables SNI.\n");
+  }
 
   /* Disable cipher suites that ST supports but are not safe. These ciphers
      are unlikely to be used in any case since ST gives other ciphers a much