libcgroup: Introduce an API to get the mount point of a specific subsystem

libcgroup: Introduce an API to get the mount point of a specific subsystem

Introduce an API which will query the mount table and return the mount point
of a specific subsystem. This is needed in the case when the user knows which
subsystem he wants the details of, which would make the use of the get_controller*
APIs cumbersome.

Signed-off-by: Dhaval Giani <dhaval@linux.vnet.ibm.com>