Improved the chdir() code:

- Renamed push_dir() to change_dir() and revised it a little so that it
  can chdir() to a relative path without an intervening chdir() back to
  the staring path.
- Renamed push_pathname() to change_pathname() and revised it to take
  different args and to only call path_is_daemon_excluded() on a new
  path (not a revisit of a file's already-checked path).
- Fixed change_pathname() to set the right pathname value when a chdir()
  call fails.
- Set orig_dir once outside of the change_pathname() function.
- Got rid of pop_dir().