Add (naive) searching and goto line in copy mode. Searching is C-r and C-s with

emacs keys, / and ? with vi; n repeats the search again with either key
set. All searching wraps the top/bottom. Goto line is g for both emacs and vi.

The search prompts don't have full line editing, just simple append and delete
characters.

Also sort the mode keys list in tmux.1.

tmux.h

@@ -394,6 +394,7 @@ enum mode_key_cmd {
 	MODEKEYCOPY_COPYSELECTION,
 	MODEKEYCOPY_DOWN,
 	MODEKEYCOPY_ENDOFLINE,
+	MODEKEYCOPY_GOTOLINE,
 	MODEKEYCOPY_HALFPAGEDOWN,
 	MODEKEYCOPY_HALFPAGEUP,
 	MODEKEYCOPY_LEFT,
@@ -402,6 +403,9 @@ enum mode_key_cmd {
 	MODEKEYCOPY_PREVIOUSPAGE,
 	MODEKEYCOPY_PREVIOUSWORD,
 	MODEKEYCOPY_RIGHT,
+	MODEKEYCOPY_SEARCHAGAIN,
+	MODEKEYCOPY_SEARCHDOWN,
+	MODEKEYCOPY_SEARCHUP,
 	MODEKEYCOPY_STARTOFLINE,
 	MODEKEYCOPY_STARTSELECTION,
 	MODEKEYCOPY_UP,