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/cgroup.h

@@ -26,9 +26,15 @@
 
 #include <uev/uev.h>
 
+struct cgroup {
+	char name[16];
+	char cfg[128];
+};
+
 void cgroup_init    (uev_ctx_t *ctx);
+void cgroup_config  (size_t num, struct cgroup cg[]);
 
 int  cgroup_user    (char *name, int pid);
-int  cgroup_service (char *name, int pid);
+int  cgroup_service (char *name, int pid, struct cgroup *cg);
 
 #endif /* FINIT_CGROUP_H_ */