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>

src/test/librc.funcs.hidden.list

@@ -51,4 +51,6 @@ rc_stringlist_new
 rc_stringlist_sort
 rc_stringlist_split
 rc_sys
+rc_sys_v1
+rc_sys_v2
 rc_yesno

src/test/rc.funcs.list

@@ -104,5 +104,9 @@ rc_stringlist_split
 rc_stringlist_split@@RC_1.0
 rc_sys
 rc_sys@@RC_1.0
+rc_sys_v1
+rc_sys_v1@@RC_1.0
+rc_sys_v2
+rc_sys_v2@@RC_1.0
 rc_yesno
 rc_yesno@@RC_1.0