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/util.c

@@ -88,9 +88,10 @@ int fnwrite(char *value, char *fmt, ...)
 		return -1;
 
 	/* echo(1) always adds a newline */
-	fputs(value, fp);
-	fputs("\n", fp);
-	fclose(fp);
+	if (fputs(value, fp) == EOF ||
+	    fputs("\n", fp) == EOF  ||
+	    fclose(fp))
+		return -1;
 
 	return 0;
 }