Add support for configuring cgroups and their settings on services

This patch adds support for modifying settings for the default cgroups;
init, user, and system, as well as adding up to a total of eight groups
for the system.

Services can now be assigned to a cgroup, with optional extra settings
for that particular process group.  The syntax is slightly contrivied
but follows the overall Finit syntax of prop:value,prop':value', e.g.

   cgroup maint cpu.weight:123,mem.max:10000

Starting with the introduction of rlimits, a group of services sharing
the same .conf file can share the same (locally "global") rlimits, and
now also the same cgroup, e.g.

    cgroup.maint
    service foo
    service bar cgroup:mem.max:1000

This puts foo and bar in the same top-level cgroup 'maint', with an
extra memory restriction on bar for max 1000 bytes memory.

NOTE: 'mem.' is a Finit extension, a shorthand for cgroups2 'memory.'

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

contrib/finit.conf

@@ -8,6 +8,18 @@ module evdev
 module loop
 module psmouse
 
+# Top-level cgroups and their default settings.  All groups mandatory
+# but more can be added, max 8 groups in total currently.  The cgroup
+# 'root' is also available, reserved for RT processes.  Settings are
+# as-is, only one shorthand 'mem.' exists, other than that it's the
+# cgroup v2 controller default names.
+cgroup init   cpu.weight:100
+cgroup user   cpu.weight:100
+cgroup system cpu.weight:9700 cpu.max:50000
+
+# Example extra cgroup
+cgroup maint  cpu.weight:100
+
 # Runlevel to start after bootstrap, runlevel 'S'
 runlevel 2
 
@@ -16,7 +28,7 @@ network service networking start
 
 # Max file size for each log file: 100 kiB, rotate max 4 copies:
 # log => log.1 => log.2.gz => log.3.gz => log.4.gz
-log size=100k count=4
+log size:100k count:4
 
 # Virtual consoles to start getty on
 tty /dev/tty1
@@ -30,9 +42,24 @@ tty /dev/tty3
 #run [2] /etc/init.d/networking start      -- Start networking
 
 # Services to be monitored and respawned as needed
+
+# klgod and syslogd are placed in the maint cgroup
+cgroup.maint
 service [2345] <pid/syslogd> klogd -n     -- Kernel logging server
 service [2345] syslogd -n                 -- Syslog server
-service [3]    gdm                        -- GNOME Display Manager
+
+# gdm in the system cgroup (default when read from it's own .conf file)
+cgroup.system
+service [3] cgroup:cpu.weight:250,mem.max:10M gdm -- GNOME Display Manager
+
+# Start SSH daemon with opts from /etc/default/ssh (if available), move
+# to user cgroup with cpu.weight:250.  Default for services is system
+# group.  Default weight in user group is 100 (above).  Max length of
+# cgroup argument is (currently) 63 chars.
+#
+# Notice alternative syntax where current cgroup is only set for this
+# particular service.
+service [2345] cgroup.user:cpu.weight:250,mem.max:10M env:-/etc/default/ssh /usr/sbin/sshd -D $SSHD_OPTS -- OpenSSH daemon
 
 # The BusyBox ntpd does not use syslog when running in the foreground
 # So we use this trick to redirect stdout/stderr to a log file.  The