core: add @system special value to ConditionUser=

It allows checking if the user is a system user or a normal user

src/shared/condition.c

@@ -154,6 +154,9 @@ static int condition_test_user(Condition *c) {
         if (r >= 0)
                 return id == getuid() || id == geteuid();
 
+        if (streq("@system", c->parameter))
+                return getuid() <= SYSTEM_UID_MAX || geteuid() <= SYSTEM_UID_MAX;
+
         username = getusername_malloc();
         if (!username)
                 return -ENOMEM;

src/test/test-condition.c

@@ -385,6 +385,16 @@ static void test_condition_test_user(void) {
         log_info("ConditionUser=%s → %i", username, r);
         assert_se(r == 0);
         condition_free(condition);
+
+        condition = condition_new(CONDITION_USER, "@system", false, false);
+        assert_se(condition);
+        r = condition_test(condition);
+        log_info("ConditionUser=@system → %i", r);
+        if (geteuid() == 0)
+                assert_se(r > 0);
+        else
+                assert_se(r == 0);
+        condition_free(condition);
 }
 
 static void test_condition_test_group(void) {