New function num-processors

New function num-processors

This addresses a FIXME comment in lisp/emacs-lisp/comp.el,
relating to the number of subsidiary processes used by
comp-run-async-workers in native compilation.
* admin/merge-gnulib (GNULIB_MODULES): Add nproc.
* doc/lispref/processes.texi (Process Information), etc/NEWS:
Document num-processors.
* lib/gnulib.mk.in, m4/gnulib-comp.m4: Regenerate.
* lib/nproc.c, lib/nproc.h, m4/nproc.m4:
New files, copied from Gnulib by admin/merge-gnulib.
* lisp/emacs-lisp/comp.el (w32-get-nproc): Remove decl.
(comp-effective-async-max-jobs): Use num-processors.
* src/process.c: Include nproc.h.
(Fnum_processors): New function.
(syms_of_process): Define ‘all’, ‘current’, ‘num-processors’.
* src/w32proc.c (Fw32_get_nproc): Add FIXME comment.
* test/src/process-tests.el (process-num-processors): New test.