tools/cgget: add '-c' option

tools/cgget: add '-c' option

Add '-c' option to cgget, that prints the controllers and their
versions,
this is useful for the user, who wants to list the available controllers
and/or their version too.

 $ cgget -c
 #Controller     Version
 cpu             2
 memory          2
 cpuset          1
 misc            1
 hugetlb         1
 blkio           1
 net_cls         1
 net_prio        1
 freezer         1
 devices         1
 pids            1
 perf_event      1
 rdma            1

this option works with legacy, unified, and hybrid cgroup setup modes.

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>
TJH: Changed comment formatting