user-util: add new uid_is_system() helper

This adds uid_is_system() and gid_is_system(), similar in style to
uid_is_dynamic(). That a helper like this is useful is illustrated by
the fact that test-condition.c didn't get the check right so far, which
this patch fixes.

src/test/test-condition.c

@@ -391,7 +391,7 @@ static void test_condition_test_user(void) {
         assert_se(condition);
         r = condition_test(condition);
         log_info("ConditionUser=@system → %i", r);
-        if (getuid() < SYSTEM_UID_MAX || geteuid() < SYSTEM_UID_MAX)
+        if (uid_is_system(getuid()) || uid_is_system(geteuid()))
                 assert_se(r > 0);
         else
                 assert_se(r == 0);