Merge branch 'rs/qsort-s'

A few codepaths had to rely on a global variable when sorting
elements of an array because sort(3) API does not allow extra data
to be passed to the comparison function.  Use qsort_s() when
natively available, and a fallback implementation of it when not,
to eliminate the need, which is a prerequisite for making the
codepath reentrant.

* rs/qsort-s:
ref-filter: use QSORT_S in ref_array_sort()
string-list: use QSORT_S in string_list_sort()
perf: add basic sort performance test
add QSORT_S
compat: add qsort_s()

diff --cc Makefile
Simple merge

diff --cc ref-filter.c
Simple merge