Implement the Opaque PRF Input TLS extension

(draft-rescorla-tls-opaque-prf-input-00.txt), and do some cleanups and
bugfixes on the way.  In particular, this fixes the buffer bounds
checks in ssl_add_clienthello_tlsext() and in ssl_add_serverhello_tlsext().

Note that the opaque PRF Input TLS extension is not compiled by default;
see CHANGES.

apps/s_server.c

@@ -1575,6 +1575,11 @@ static int sv_body(char *hostname, int s, unsigned char *context)
 						 strlen((char *)context));
 	}
 	SSL_clear(con);
+#if 0
+#ifdef TLSEXT_TYPE_opaque_prf_input
+	SSL_set_tlsext_opaque_prf_input(con, "Test server", 1);
+#endif
+#endif
 
 	if (SSL_version(con) == DTLS1_VERSION)
 		{