nproc: support OMP_THREAD_LIMIT to set a max value

nproc: support OMP_THREAD_LIMIT to set a max value

This comes from the latest gnulib.
Also handling of OMP_NUM_THREADS has been adjusted
to support comma separated values indicating a nesting level,
in which case the first value is taken.  Also OMP_NUM_THREADS=0
is now ignored instead of being treated as 1, to match
the behavior of libgomp.

* NEWS: Mention the OMP_THREAD_LIMIT improvement,
and OMP_NUM_THREADS now handling nested values.
* doc/coreutils.texi (nproc invocation): Describe OMP_THREAD_LIMIT
as a way to set the max value, with OMP_THREAD_LIMIT setting the min.
* tests/misc/nproc-override.sh: A new test to exercise the
updated gnulib code with all combinations of these OMP variables.
* tests/local.mk: Reference the new test.