api.c: cache cgrules with CGFLAG_USECACHE flag

api.c: cache cgrules with CGFLAG_USECACHE flag

pam_cgroup.so plugin uses /etc/cgrules.conf to assign processes
to the requested cgroup.  This works well with active
cgrulesengd but in the instances where cgrulesengd is disabled,
the rules are not read and cached by default. Fix this is by
reloading and caching the rules when called with CGFLAG_USECACHE
flag.

Fixes: https://github.com/libcgroup/libcgroup/issues/84
Reported-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>
TJH: Minor commit comment changes