ftdetect: Add autocmds for other types of systemd units
ftdetect: Load systemd syntax for service units outside of systemd directories

Some projects actually have their own systemd units that they install
that aren't located in a path that matches */systemd/*.service. While
this isn't very common for other types of units, I think it's common
enough for service units that we should consider files outside of the
systemd directories.
Merge pull request #1 from Lyude/master

Improvements to ftdetect