Add systemd user service file for canto-daemon

Will be placed in /usr/lib/systemd/user by setup.py

Can do `systemctl --user [enable|start|stop] canto-daemon`

This is now the *correct* way to start canto-daemon, as it's the only
way that it gets shutdown cleanly thanks to Python's slow SIGTERM
handling.

NOTE: Make sure `localectl status` or /etc/locale.conf looks sane as
this will be the locale services are run in. The daemon doesn't like
going to an ASCII locale if you've already been running it in UTF-8.
Shutdown tweaks

- Differentiate TERM and INT
- Stop disk IO faster
- Make feed.index() callers bail without initiating more disk IO when
  stopping.

This, along with the systemd service, are the unfortunate consequence of
Python taking its sweet fucking time actually doing the SIGTERM handler.
I couldn't even get a log.info out before getting SIGKILL'd. So, the
service gives the daemon enough time to shutdown and, otherwise try to
get it done faster.