Change the way the working directory for new processes is discovered. If

Change the way the working directory for new processes is discovered. If
default-path isn't empty, it is used. Otherwise:

1) If tmux neww is run from the command line, the working directory of the
   client is used.

2) Otherwise sysctl KERN_PROC_CWD is used to retrieve the current
   working directory of the process in the active pane.

3) If that fails, the directory where the session was created is used.

Support code by Romain Francois, OpenBSD specific bits by me.

Note this requires a recent userland and kernel with KERN_PROC_CWD.