Added check for libnl headers in configure.ac.

Review: https://reviews.apache.org/r/23132
Enabled the support for creating nested cgroup structures. This is
important for systemd support.

Review: https://reviews.apache.org/r/22977
Do not append endln on cgroup write.

Minor update to remove the endln appending to cgroup writes. This
causes issues in later kernels, I uncovered it in rawhide.

Review: https://reviews.apache.org/r/22979
Added documentation for authorization.

Review: https://reviews.apache.org/r/23104