api.c: Add function to read the cgroup.subtree_control file

api.c: Add function to read the cgroup.subtree_control file

Add a function, cgroupv2_get_subtree_control(), that verifies
if a controller is enabled in the cgroup.subtree_control file
for the requested path.

This function will be used in the next commit - "api.c: Add function
to check if a controller is enabled for a cgroup v2 cgroup" - to
determine if the requested cgroup v2 controller is enabled for a
cgroup.  This is a required check to ensure that the
cgclassify/cgexec operation will succeed.

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