init: Rename systemd units

Use strongswan-starter for the legacy unit and simply strongswan for the
modern one (strongswan-swanctl is configured as alias, which should
cause the installation of symlinks when the service is enabled via
systemctl).

init/systemd/Makefile.am

@@ -8,5 +8,4 @@ strongswan.service : strongswan.service.in
 	$(AM_V_GEN) \
 	sed \
 	-e "s:@SBINDIR@:$(sbindir):" \
-	-e "s:@IPSEC_SCRIPT@:$(ipsec_script):" \
 	$(srcdir)/$@.in > $@