Support cpu usage management for Linux

This commit was modified by William Hubbs as follows:

- The paths in the cgroup fs were put into variables to ease
  maintenance.
- Documentation was added to rc.conf.Linux.
- The services were added originally to openrc/svcname cgroups under the
  controller cgroups, but this left an "openrc" cgroup which was unused.
  Now they are added to individual cgroups with the name openrc_${RC_SVCNAME}.

etc/rc.conf.Linux

@@ -20,10 +20,21 @@
 # consolefont, numlock, etc ...)
 rc_tty_number=12
 
+##############################################################################
+# CGROUPS RESOURCE MANAGEMENT
+
 # If you have cgroups turned on in your kernel, this switch controls
 # whether or not a group for each controler is mounted under
 # /sys/fs/cgroup.
-# Support for process management by cgroups is planned in the future,
-# so if you turn this off, be aware that you may not be able to use that
-# feature.
+# None of the other options in this section work if this is set to "NO".
 #rc_controller_cgroups="YES"
+
+# These options can be set globally in this file; however, if you do
+# this, the same setting will apply to all of your services.
+# If you want different settings for each service, place the settings in
+# /etc/conf.d/foo for service foo.
+
+# This is the number of CPU shares a service is allowed to use. The
+# default value, set by the kernel, is 1024.
+# Also, this setting is not preserved if you suspend your computer.
+#rc_cgroup_cpu_shares=1024