[mod_mysql_vhost] deprecated; use mod_vhostdb_mysql

add warning at server startup when mod_mysql_vhost is loaded

mod_vhostdb_mysql subsumes mod_mysql_vhost.  Individual mod_mysql_vhost
directives map one-to-one to keywords in vhostdb.mysql = (...) directive

src/mod_mysql_vhost.c

@@ -198,6 +198,10 @@ SETDEFAULTS_FUNC(mod_mysql_vhost_set_defaults) {
         T_CONFIG_SCOPE_UNSET }
     };
 
+    log_error(srv->errh, __FILE__, __LINE__,
+      "mod_mysql_vhost is deprecated and will be removed in a future version; "
+      "please migrate to use mod_vhostdb_mysql");
+
     plugin_data * const p = p_d;
     if (!config_plugin_values_init(srv, p, cpk, "mod_mysql_vhost"))
         return HANDLER_ERROR;