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/rc.conf.in

@@ -115,3 +115,4 @@
 # You can also remove dependencies.
 # This is mainly used for saying which servies do NOT provide net.
 #rc_net_tap0_provide="!net"
+