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.Linux

@@ -1,3 +1,15 @@
+# This is the subsystem type. Valid options on Linux:
+# ""        - nothing special
+# "lxc"     - Linux Containers
+# "openvz"  - Linux OpenVZ
+# "prefix"  - Prefix
+# "uml"     - Usermode Linux
+# "vserver" - Linux vserver
+# "xen0"    - Xen0 Domain
+# "xenU"    - XenU Domain
+# If unset, the old automagic detection code will be triggered. Said old code
+# is deprecated and be removed not later than 2010/03/01.
+rc_sys=""
 
 ##############################################################################
 # LINUX SPECIFIC OPTIONS