getDTthreads more robust

[mattdowle]

Attempt 1 for #2707

If user calls setDTthread(0) it was calling omp_set_num_threads() to omp_get_thread_limit() which was INT_MAX by default. Relatively new fsort was using DTthreads without restriction, whereas usage elsewhere was typically limiting number of threads based on chunk size or similar.
All usage of omp_set.* removed and the grep in CRAN_Release.cmd expanded to include openmp-utils.c, too.

[codecov-io]

Codecov Report

Merging #2708 into master will decrease coverage by <.01%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #2708      +/-   ##
==========================================
- Coverage   93.33%   93.32%   -0.01%     
==========================================
  Files          61       61              
  Lines       12220    12225       +5     
==========================================
+ Hits        11405    11409       +4     
- Misses        815      816       +1

Impacted Files	Coverage Δ
R/openmp-utils.R	100% <100%> (ø)	⬆️
src/openmp-utils.c	88.88% <84.61%> (-2.03%)	⬇️

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