Install systemd service under PREFIX

Andre Toerienrequested to merge
AThePeanut4/OpenRGB:systemd-service-path into master

Currently the systemd service file is installed into /etc/systemd/system, which is the location meant for locally-created unit files, not for those provided by programs.

These should be installed in PREFIX/lib/systemd/system (so if PREFIX is e.g. /usr, then /usr/lib/systemd/system).

NB: I see fedora/OpenRGB.spec.in has /etc/systemd/system/openrgb.service, I assume this should be changed as well, but I don't know how Fedora packaging works so I've left it alone.

Sidenote: is there a reason why the default service file uses RemainAfterExit=yes? The one Arch ships (see #3798) does not have it, and I don't see why it is needed. Also, that issue can presumably be closed.

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