makefile: change install to install everything

`make install` should install everything, then 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

Makefile

-install:
+install: install_openrc install_systemd
 	install -Dm 755 src/swclock-offset-boot.sh \
 		$(DESTDIR)$(PREFIX)/bin/swclock-offset-boot
 	install -Dm 755 src/swclock-offset-shutdown.sh \