Add systemd service unit

[tlvince]

This adds a basic systemd.service unit for redsocks.

I maintain the redsocks package for Arch Linux, which is transitioning away
from sysvinit to systemd. Although I'm new to systemd, it is my
understanding that one advantage of it is it allows for rc scripts to be
maintained upstream rather than multiple idiosyncratic scripts per distribution.

Note, I labelled it 'basic' as it also highlights a feature request for
redsocks: a -u, --user option that allows redsocks to drop root privileges
and switch to the given userid. This will mean su can be removed and simplify
the service file.

[darkk]

Well, I assume you're wrong about mainline maintenance. E.g. ubuntu have chosen upstart init replacement instead of systemd, so... :) I can include systemd.service, but I can't properly maintain as soon as I don't have Arch installation.

As far as I know, it's bad practice to use su in init scripts. The reason is described here: http://upstart.ubuntu.com/cookbook/#changing-user

su(1) must fork to be able to hold its PAM session open

On the other hand it matters iff PAM is broken as redsocks forks and su will close PAM session after second fork. Also, redsocks has to be started as root for UDP redirection to work and it probably should have non-forking option for systemd/upstart init systems to have cleaner restart-on-crash policy.