Cache user from getpwuid because it can be very expensive on some

platforms. From Ben Maurer in GitHub issue 4973.

server-client.c

@@ -537,6 +537,7 @@ server_client_free(__unused int fd, __unused short events, void *arg)
 
 	if (c->references == 0) {
 		free((void *)c->name);
+		free((void *)c->user);
 		free(c);
 	}
 }