api.c: add support to build procs path for cgroup.thread

api.c: add support to build procs path for cgroup.thread

In cgroup_build_tid_path(), add support for building path to write
tid into cgroup.thread instead of cgroup.procs, if the calling function
is cgroup_attach_thread_tid() or else build path to write into
cgroup.procs.

Fixes: https://github.com/libcgroup/libcgroup/issues/441
Reported-by: Adriaan Schmidt <adriaan.schmidt@siemens.com>
Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>