util: define initializer for 'struct ucred' that properly invalidates all fields

i.e. let's make sure to invalid uid/gid to UID_INVAID + GID_INVALID
instead of zero.

src/basic/socket-util.h

@@ -327,3 +327,6 @@ static inline int socket_set_recvfragsize(int fd, int af, bool b) {
 }
 
 int socket_get_mtu(int fd, int af, size_t *ret);
+
+/* an initializer for struct ucred that initialized all fields to the invalid value appropriate for each */
+#define UCRED_INVALID { .pid = 0, .uid = UID_INVALID, .gid = GID_INVALID }