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

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

Enhance the cgxget 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*      cgrp1
		|
	 systemd.scope
	        |
	    cgrp-d1

* default system delegation slice/scope - read from cgconfig.conf
$ sudo cgxget -1 -r cpu.shares 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 cgxget -1 -r cpu.shares /cgrp1

or use the newly introduced -b switch to ignore the systemd
slice/scope:
$ sudo cgxget -b -1 -r cpu.shares /cgrp1

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