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/basic/user-util.h

@@ -64,6 +64,14 @@ static inline bool uid_is_dynamic(uid_t uid) {
         return DYNAMIC_UID_MIN <= uid && uid <= DYNAMIC_UID_MAX;
 }
 
+static inline bool uid_is_system(uid_t uid) {
+        return uid <= SYSTEM_UID_MAX;
+}
+
+static inline bool gid_is_system(gid_t gid) {
+        return gid <= SYSTEM_GID_MAX;
+}
+
 /* The following macros add 1 when converting things, since UID 0 is a valid UID, while the pointer
  * NULL is special */
 #define PTR_TO_UID(p) ((uid_t) (((uintptr_t) (p))-1))