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/Makefile.am

@@ -3,12 +3,12 @@ SUBDIRS =
 
 if USE_LEGACY_SYSTEMD
 if USE_CHARON
-  SUBDIRS += systemd
+  SUBDIRS += systemd-starter
 endif
 endif
 
 if USE_SYSTEMD
 if USE_SWANCTL
-  SUBDIRS += systemd-swanctl
+  SUBDIRS += systemd
 endif
 endif