Allow control characters to be entered at the command prompt prefixed

with with C-v, from  Alexander Arch in GitHub issue 4206.

tmux.h

@@ -1938,6 +1938,7 @@ struct client {
 #define PROMPT_NOFORMAT 0x8
 #define PROMPT_KEY 0x10
 #define PROMPT_ACCEPT 0x20
+#define PROMPT_QUOTENEXT 0x40
 	int			 prompt_flags;
 	enum prompt_type	 prompt_type;
 	int			 prompt_cursor;