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 use some platform specific code 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.
Idea and support code, Linux, Solaris, FreeBSD bits by Romain Francoise,
OpenBSD bits by me.