Replace the genericly-named tls.c with a tls_gnutls.c for switching

support in and out. It's easier for the build system if we do this
with #ifdef's but conditional compilation is easier while we're
playing around.

Also rename a function.

src/tls.h

@@ -27,7 +27,7 @@ void tls_shutdown(void);
 tls_t *tls_new(xmpp_ctx_t *ctx, sock_t sock);
 void tls_free(tls_t *tls);
 
-int tls_setcredentials(tls_t *tls, const char *cafilename);
+int tls_set_credentials(tls_t *tls, const char *cafilename);
 
 int tls_start(tls_t *tls);
 int tls_stop(tls_t *tls);

src/tls_gnutls.c

@@ -72,7 +72,7 @@ void tls_free(tls_t *tls)
     xmpp_free(tls->ctx, tls);
 }
 
-int tls_setcredentials(tls_t *tls, const char *cafilename)
+int tls_set_credentials(tls_t *tls, const char *cafilename)
 {
     int err;