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.

server-fn.c

@@ -56,8 +56,8 @@ server_write_ready(struct client *c)
 }
 
 int
-server_write_client(
-    struct client *c, enum msgtype type, const void *buf, size_t len)
+server_write_client(struct client *c, enum msgtype type, const void *buf,
+    size_t len)
 {
 	struct imsgbuf	*ibuf = &c->ibuf;
 	int              error;
@@ -73,8 +73,8 @@ server_write_client(
 }
 
 void
-server_write_session(
-    struct session *s, enum msgtype type, const void *buf, size_t len)
+server_write_session(struct session *s, enum msgtype type, const void *buf,
+    size_t len)
 {
 	struct client	*c;
 	u_int		 i;