Fix #188: support without cgroups v2 support

This patch adds support for detecting if the Linux kernel Fint runs on
has support for cgroups v2.  The probe is done very early, in runlevel
S, when attempting to mount /sys/fs/cgroup.  Meaning, the log message
may be lost on systems that do not run sysklogd[1].

Here's the relevant logs from a system running myLinux on a kernel
that has cgroups v2 support disabled:

    Jan 10 20:19:40 anarchy finit[1]: myLinux 2021.11-696-g8fddaf36ed, entering runlevel S
    Jan 10 20:19:40 anarchy finit[1]: Kernel does not support cgroups v2, disabling.
    Jan 10 20:19:41 anarchy finit[1]: myLinux 2021.11-696-g8fddaf36ed, entering runlevel 2

All code paths that attempt to set or query kernel cgroups config has
been short circuited when this happens.  However, this does not apply
to any cgroup related config in the Finit .conf files -- which may of
course be omitted.  Yet, if configuration directives exist, they will
be parsed and may cause error.

Also, the initctl tool checks if /sys/fs/cgroup is mounted at runtime
and disables all status output and commands that rely on the feature.

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

src/cgutil.c

@@ -53,6 +53,10 @@ uint64_t total_ram;			/* From sysinfo() */
 struct cg  dummy;			/* empty result "NULL"      */
 struct cg *list;
 
+int cgroup_avail(void)
+{
+	return fismnt(FINIT_CGPATH);
+}
 
 char *pid_cmdline(int pid, char *buf, size_t len)
 {