GSoC 2018 Syntax and semantics of systemd units in the Shepherd - 1st update

[Ioannis Panagiotis Koutsidis]

Hi Guix!

As the 1st phase is coming to an end I decided to post my progress. I have
implemented the unit file parsing as well as some of the basic entries supported
by it, such as ExecStart, User, Group, Restart, etc. In addition, support for
the systemd Restart values (on-success, on-failure, on-abnormal, and on-abort)
was added to the Shepherd via the restart-systemd field in the <service> class,
letting services written in guile to also use that feature.

During the next phases I will focus on other common .service entries, .socket
support, as well as thoroughly testing the code.

patch
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