nproc: support nested OMP_NUM_THREADS, and OMP_THREAD_LIMIT
* lib/nproc.c (parse_omp_threads): A new function refactored
from num_processors() to support parsing both of the
above environment variables.
(num_processors): Prefer using omp_get_num_threads() with [_OPENMP]
to accurately reflect the current OpenMP nesting level.
Also support the OMP_THREAD_LIMIT environment variable
to limit the max value determined from OMP_NUM_THREADS.
* modules/nproc: Depend on minmax header.
Suggested by Oliver Heimlich.