Alter how tmux handles the working directory to internally use file

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.