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/conf.h

@@ -24,16 +24,22 @@
 #ifndef FINIT_CONF_H_
 #define FINIT_CONF_H_
 
+#include "cgroup.h"
 #include "svc.h"
 
 extern int logfile_size_max;
 extern int logfile_count_max;
 
 extern struct rlimit global_rlimit[];
+extern struct cgroup cgroups[];
+extern size_t cgroup_num;
+extern char cgroup_current[];
 
 int   str2rlim(char *str);
 char *rlim2str(int rlim);
 
+struct cgroup *conf_cgfind(char *name);
+
 int  conf_init            (uev_ctx_t *ctx);
 void conf_reload          (void);
 int  conf_any_change      (void);