nss-systemd: tweak checks when we consult PID 1 for dynamic UID/GID lookups

Instead of contacting PID 1 for dynamic UID/GID lookups for all
UIDs/GIDs that do not qualify as "system" do the more precise check
instead: check if they actually qualify for the "dynamic" range.

src/basic/user-util.h

@@ -64,6 +64,10 @@ static inline bool uid_is_dynamic(uid_t uid) {
         return DYNAMIC_UID_MIN <= uid && uid <= DYNAMIC_UID_MAX;
 }
 
+static inline bool gid_is_dynamic(gid_t gid) {
+        return uid_is_dynamic((uid_t) gid);
+}
+
 static inline bool uid_is_system(uid_t uid) {
         return uid <= SYSTEM_UID_MAX;
 }