user-util: add UID_NOBODY defines that resolve to (uid_t) 65534

We use it all over the place, let's add a #define for it. Makes things
easier greppable, and more explanatory I think.

src/basic/user-util.h

@@ -60,6 +60,9 @@ int take_etc_passwd_lock(const char *root);
 #define UID_INVALID ((uid_t) -1)
 #define GID_INVALID ((gid_t) -1)
 
+#define UID_NOBODY ((uid_t) 65534U)
+#define GID_NOBODY ((gid_t) 65534U)
+
 static inline bool uid_is_dynamic(uid_t uid) {
         return DYNAMIC_UID_MIN <= uid && uid <= DYNAMIC_UID_MAX;
 }