Switch the prompt code to return an empty string when the user enters no

response and reserve NULL for an explicit cancel. Change all callbacks to treat
them the same so no functional change.

Also add cancel key bindings to emacs mode which were missing.

cmd-command-prompt.c

@@ -112,7 +112,7 @@ cmd_command_prompt_callback(void *data, const char *s)
 	char				*cause, *ptr, *buf, ch;
 	size_t				 len, slen;
 
-	if (s == NULL)
+	if (s == NULL || *s == '\0')
 		return (0);
 	slen = strlen(s);