Implement explicit selection of subsystem types.

- Fixes bugs #347583, #349389, both of which were triggered by cgroups
  being detected as the LXC subsystem type.
- Makes it much easier to select "prefix" type.
- "rc -S" will now print a warning if you have not configured rc_sys in
  /etc/rc.conf
- All other semantics of rc_sys are unchanged in this patch.

Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>

etc/Makefile

@@ -7,3 +7,6 @@ MK=	../mk
 include ${MK}/os.mk
 include Makefile.${OS}
 include ${MK}/scripts.mk
+
+rc.conf: rc.conf.in rc.conf.${OS}
+	${SED} ${SED_REPLACE} ${SED_EXTRA} $^ > $@