Use exception-based approach for cgroup/ulimit setup

Note from William Hubbs:
I spoke with Roy about this, and he pointed out that user-defined
functions may need the limits applied, so it is better to go with a
method that uses exceptions to determine which functions apply the
limits.

X-Gentoo-Bug: 522408
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=522408