api.c: Add function to read /proc/{pid}/cgroup

api.c: Add function to read /proc/{pid}/cgroup

This commit adds a function that, given a pid, can read in
the controllers and cgroups listed in /proc/{pid}/cgroup.
The caller is expected to allocate the controller_list[]
and cgroup_list[] arrays as well as null each entry in the
arrays.  cg_get_cgroups_from_proc_cgroups() will allocate
the necessary memory for each string within the arrays.

Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>