Expose a more consistent subset of systemd's service directives

[jdstrand]

@hcochran

Ditto. We do not like running our services as root but wish to use a system user. I am neutral about whether all we need is support for the User= and Group= directives, vs. launching an instance of systemd --user. The former seems simpler.

I’m in the process of allowing snaps to privilege drop to the ‘daemon’ user/group. After that, we’ll be implementing support for using other snapd-managed users. Perhaps User= and Group= could be added after that, but I’d prefer these directives not be added until this other feature is implemented.