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

platforms. From Ben Maurer in GitHub issue 4973.

tmux.h

@@ -1958,6 +1958,7 @@ typedef void (*overlay_resize_cb)(struct client *, void *);
 struct client {
 	const char		*name;
 	struct tmuxpeer		*peer;
+	const char		*user;
 	struct cmdq_list	*queue;
 
 	struct client_windows	 windows;