Fix shebang of `auth.py`.

The shebang now reflects the fact that `auth.py` is intended to run
equally well on Python 2 and on Python 3.
Read path information from `~/.profile`.

Note that I use a bash function to simplify appending to the PATH
variable, and that this reading relies on said function.
Make a couple of minor improvements.

- The Perl script that is now used can read session info from both
  ConsoleKit and `loginctl`.
- GPG info is now read from `keychain` just like SSH info, and also with
  a more robust regex.