tools/cgget: add support for default systemd delegation slice/scope

tools/cgget: add support for default systemd delegation slice/scope

Enhance the cgget tool to support default systemd delegation
slice/scope, if set in the cgconfig.conf's systemd setting.
Parse the configuration file and read the systemd::delegate setting,
if any, and set it as the default systemd slice/scope in
systemd_default_cgroup. Setting it appends the slice/scope name to
the constructed default cgroup mount path.

When the user passes the relative cgroup name, its appends to the
sub-tree (delegated slice/scope) by default and if the user wishes to get
details about the cgroups in another subtree or cgroup root hierarchy,
they need to use the absolute path. For example:
		  cgroup_root
		 /	    \
		/	     \
	 systemd.slice[1]      cgrp1
		|
	 systemd.scope
		|
	    cgrp-d1

[1] default systemd delegation slice/scope - read from cgconfig.conf

$ sudo cgget -g cpu:cgrp-d1

will display the CPU controller information under the
cgroup_root:systemd_default_cgroup:cgrp-d1

and for viewing the CPU controller information of cgrp1, the user can
use:
$ sudo cgget -g cpu:/cgrp1

or use the newly introduced -b switch to ignore the systemd slice/scope:
$ sudo cgget -b -g cpu:cgrp

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>
TJH: Fix minor typo in a comment where delegate was written instead
     of setdefault
(cherry picked from commit aa9c756add6f1ca9b61cb54ce1334d6b0be745b9)