Invent qsort_interruptible().

Justin Pryzby reported that some scenarios could cause gathering
of extended statistics to spend many seconds in an un-cancelable
qsort() operation.  To fix, invent qsort_interruptible(), which is
just like qsort_arg() except that it will also do CHECK_FOR_INTERRUPTS
every so often.  This bloats the backend by a couple of kB, which
seems like a good investment.  (We considered just enabling
CHECK_FOR_INTERRUPTS in the existing qsort and qsort_arg functions,
but there are some callers for which that'd demonstrably be unsafe.
Opt-in seems like a better way.)

For now, just apply qsort_interruptible() in statistics collection.
There's probably more places where it could be useful, but we can
always change other call sites as we find problems.

Back-patch to v14.  Before that we didn't have extended stats on
expressions, so that the problem was less severe.  Also, this patch
depends on the sort_template infrastructure introduced in v14.

Tom Lane and Justin Pryzby

Discussion: https://postgr.es/m/20220509000108.GQ28830@telsasoft.com

src/backend/statistics/mvdistinct.c

@@ -489,8 +489,8 @@ ndistinct_for_combination(double totalrows, StatsBuildData *data,
 	}
 
 	/* We can sort the array now ... */
-	qsort_arg((void *) items, numrows, sizeof(SortItem),
-			  multi_sort_compare, mss);
+	qsort_interruptible((void *) items, numrows, sizeof(SortItem),
+						multi_sort_compare, mss);
 
 	/* ... and count the number of distinct combinations */