Expand usage of xfsm_verbose for debugging

This patch:
1) Documents the XFSM_VERBOSE environment variable in the
   man page.
2) Changes xfsm_verbose's output from a timestamp to the usual func,
   file, line, message format used in other Xfce programs.
3) Uses xfsm_verbose in more places to help aid in debugging.
4) Saves the last xfsm run in a .last extension, so there's always
   the current session log and the last one to compare to.