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>

src/svc.h

@@ -32,6 +32,7 @@
 #include <lite/queue.h>		/* BSD sys/queue.h API */
 #include <uev/uev.h>
 
+#include "cgroup.h"
 #include "helpers.h"
 
 typedef int svc_cmd_t;
@@ -94,6 +95,7 @@ typedef struct svc {
 
 	/* Limits and scoping */
 	struct rlimit  rlimit[RLIMIT_NLIMITS];
+	struct cgroup  cgroup;
 
 	/* Service details */
 	int            sighalt;        /* Signal to stop prorcess, default: SIGTERM */