userdb: add new flag for excluding varlink data in lookups

This is useful to later-on use the userdb infra for only some sources.

src/shared/userdb.c

@@ -402,6 +402,9 @@ static int userdb_start_query(
         assert(iterator);
         assert(method);
 
+        if (FLAGS_SET(flags, USERDB_EXCLUDE_VARLINK))
+                return -ENOLINK;
+
         e = getenv("SYSTEMD_BYPASS_USERDB");
         if (e) {
                 r = parse_boolean(e);

src/shared/userdb.h

@@ -17,6 +17,7 @@ DEFINE_TRIVIAL_CLEANUP_FUNC(UserDBIterator*, userdb_iterator_free);
 typedef enum UserDBFlags {
         /* The main sources */
         USERDB_EXCLUDE_NSS          = 1 << 0,  /* don't do client-side nor server-side NSS */
+        USERDB_EXCLUDE_VARLINK      = 1 << 1,  /* don't talk to any varlink services */
 
         /* Modifications */
         USERDB_SUPPRESS_SHADOW      = 1 << 3,  /* don't do client-side shadow calls (server side might happen though) */