add QSORT_S

add QSORT_S

Add the macro QSORT_S, a convenient wrapper for qsort_s() that infers
the size of the array elements and dies on error.

Basically all possible errors are programming mistakes (passing NULL as
base of a non-empty array, passing NULL as comparison function,
out-of-bounds accesses), so terminating the program should be acceptable
for most callers.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>