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 ~

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.

session.c

@@ -85,9 +85,8 @@ session_find_by_id(u_int id)
 
 /* Create a new session. */
 struct session *
-session_create(const char *name, const char *cmd, const char *cwd,
-    struct environ *env, struct termios *tio, int idx, u_int sx, u_int sy,
-    char **cause)
+session_create(const char *name, const char *cmd, int cwd, struct environ *env,
+    struct termios *tio, int idx, u_int sx, u_int sy, char **cause)
 {
 	struct session	*s;
 
@@ -99,7 +98,7 @@ session_create(const char *name, const char *cmd, const char *cwd,
 		fatal("gettimeofday failed");
 	session_update_activity(s);
 
-	s->cwd = xstrdup(cwd);
+	s->cwd = dup(cwd);
 
 	s->curw = NULL;
 	TAILQ_INIT(&s->lastw);
@@ -171,7 +170,7 @@ session_destroy(struct session *s)
 		winlink_remove(&s->windows, wl);
 	}
 
-	free(s->cwd);
+	close(s->cwd);
 
 	RB_INSERT(sessions, &dead_sessions, s);
 }
@@ -227,8 +226,8 @@ session_previous_session(struct session *s)
 
 /* Create a new window on a session. */
 struct winlink *
-session_new(struct session *s,
-    const char *name, const char *cmd, const char *cwd, int idx, char **cause)
+session_new(struct session *s, const char *name, const char *cmd, int cwd,
+    int idx, char **cause)
 {
 	struct window	*w;
 	struct winlink	*wl;
@@ -251,8 +250,8 @@ session_new(struct session *s,
 		shell = _PATH_BSHELL;
 
 	hlimit = options_get_number(&s->options, "history-limit");
-	w = window_create(
-	    name, cmd, shell, cwd, &env, s->tio, s->sx, s->sy, hlimit, cause);
+	w = window_create(name, cmd, shell, cwd, &env, s->tio, s->sx, s->sy,
+	    hlimit, cause);
 	if (w == NULL) {
 		winlink_remove(&s->windows, wl);
 		environ_free(&env);