Improve command prompt completion:

- Show a menu with completions if there are multiple.

- Don't complete argument stuff (options, layouts) at start of text.

- For -t and -s, if there is no : then complete sessions but if there is a :,
  show a menu of all windows in the session rather than trying to complete the
  window name which is a bit useless if there are duplicates.

Lots of scope for being more sophisticated left here.

tmux.h

@@ -2799,6 +2799,7 @@ __dead void printflike(1, 2) fatalx(const char *, ...);
 
 /* menu.c */
 #define MENU_NOMOUSE 0x1
+#define MENU_TAB 0x2
 struct menu	*menu_create(const char *);
 void		 menu_add_items(struct menu *, const struct menu_item *,
 		    struct cmdq_item *, struct client *,