makefile: install scripts to bin instead of sbin, use PREFIX for systemd unit install location

`make install` will now install both openrc and systemd service files.

The rationale behind this change is that packaging in distros can choose
how they include/exclude stuff. E.g. in pmOS/Alpine, the -openrc abuild
split function would stuff the openrc files into -openrc, and some
future -systemd split function () would move the systemd unit files to
some -systemd subpackage

PREFIX was dropped and paths updated so that they
were compliant with a merged /usr layout.