Rename THREADS_PER_COMMAND to QBATCH_THREADS_PER_COMMAND and also export OMP_NUM_THREADS

[gdevenyi]

We should use the QBATCH prefix everywhere.

Also it seems nproc will honour OMP_NUM_THREADS which means that many apps which rely on openmp will use proper number of threads even if script writers didn't do anything with QBATCH_THREADS_PER_COMMAND

[gdevenyi]

https://www.gnu.org/software/coreutils/manual/html_node/nproc-invocation.html

[gdevenyi]

https://www.openmp.org/spec-html/5.0/openmpse58.html