systemd: move ocid.service to contrib/

In addition, remove the installation steps from the Makefile. In
general, I personally don't like projects that install service files on
`make install` -- that's the job of a package manager.

Signed-off-by: Aleksa Sarai <asarai@suse.com>

Makefile

@@ -100,11 +100,12 @@ install: all
 	install -m 644 $(wildcard docs/*.8) $(PREFIX)/share/man/man8
 	install -d $(PREFIX)/share/man/man5
 	install -m 644 $(wildcard docs/*.5) $(PREFIX)/share/man/man5
-	install -D -m 644 ocid.service $(PREFIX)/lib/systemd/system
 	install -D -m 644 ocid.conf $(DESTDIR)/etc
 
+install.systemd:
+	install -D -m 644 ocid.service $(PREFIX)/lib/systemd/system
+
 uninstall:
-	rm -f $(PREFIX)/lib/systemd/system/ocid.service
 	rm -f ${INSTALLDIR}/{ocid,ocic}
 	rm -f $(PREFIX)/libexec/ocid/{conmon,pause}
 	for i in $(wildcard docs/*.8); do \