Alter how tmux handles the working directory to internally use file descriptors

rather than strings.

- Each session still has a current working directory.

- New sessions still get their working directory from the client that created
  them or its attached session if any.

- New windows are created by default in the session working directory.

- The -c flag to new, neww, splitw allows the working directory to be
  overridden.

- The -c flag to attach let's the session working directory be changed.

- The default-path option has been removed.

To get the equivalent to default-path '.', do:

        bind c neww -c $PWD

To get the equivalent of default-path '', do:

        bind c neww -c '#{pane_current_path}'

The equivalent of default-path '~' is left as an exercise for the reader.

This also changes the client identify protocol to be a set of messages rather
than one as well as some other changes that should make it easier to make
backwards-compatible protocol changes in future.

cmd-queue.c

@@ -283,27 +283,3 @@ cmdq_flush(struct cmd_q *cmdq)
 	}
 	cmdq->item = NULL;
 }
-
-/* Get default path using command queue. */
-const char *
-cmdq_default_path(struct cmd_q *cmdq, const char *cwd)
-{
-	struct client	*c = cmdq->client;
-	struct session	*s;
-	const char	*current;
-
-	if ((s = cmd_current_session(cmdq, 0)) == NULL)
-		return (NULL);
-
-	if (cwd == NULL)
-		cwd = options_get_string(&s->options, "default-path");
-
-	if (c != NULL && c->session == NULL && c->cwd != NULL)
-		current = c->cwd;
-	else if (s->curw != NULL)
-		current = osdep_get_cwd(s->curw->window->active->fd);
-	else
-		current = NULL;
-
-	return (cmd_default_path(s->cwd, current, cwd));
-}