Allow btree comparison functions to return INT_MIN.

Allow btree comparison functions to return INT_MIN.

Historically we forbade datatype-specific comparison functions from
returning INT_MIN, so that it would be safe to invert the sort order
just by negating the comparison result.  However, this was never
really safe for comparison functions that directly return the result
of memcmp(), strcmp(), etc, as POSIX doesn't place any such restriction
on those library functions.  Buildfarm results show that at least on
recent Linux on s390x, memcmp() actually does return INT_MIN sometimes,
causing sort failures.

The agreed-on answer is to remove this restriction and fix relevant
call sites to not make such an assumption; code such as "res = -res"
should be replaced by "INVERT_COMPARE_RESULT(res)".  The same is needed
in a few places that just directly negated the result of memcmp or
strcmp.

To help find places having this problem, I've also added a compile option
to nbtcompare.c that causes some of the commonly used comparators to
return INT_MIN/INT_MAX instead of their usual -1/+1.  It'd likely be
a good idea to have at least one buildfarm member running with
"-DSTRESS_SORT_INT_MIN".  That's far from a complete test of course,
but it should help to prevent fresh introductions of such bugs.

This is a longstanding portability hazard, so back-patch to all supported
branches.

Discussion: https://postgr.es/m/20180928185215.ffoq2xrq5d3pafna@alap3.anarazel.de