nixos/slim: Implement logging to journal

The main change here is a patch of SLiM to tread a log file of
/dev/stderr specially in that it now uses std::cerr instead of a file
for logging.

This allows us to set the logfile to stderr in NixOS for the generated
SLiM configuration file and we now get logging to the systemd journal.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>