Fix power() for large inputs yet more.

 Fix power() for large inputs yet more.
 
 Buildfarm results for commit e532b1d57 reveal the error in my thinking
 about the unexpected-EDOM case.  I'd supposed this was no longer really
 a live issue, but it seems the fix for glibc's bug #3866 is not all that
 old, and we still have at least one buildfarm animal (lapwing) with the
 bug.  Hence, resurrect essentially the previous logic (but, I hope, less
 opaquely presented), and explain what it is we're really doing here.
 
 Also, blindly try to fix fossa's failure by tweaking the logic that
 figures out whether y is an odd integer when x is -inf.  This smells
 a whole lot like a compiler bug, but I lack access to icc to try to
 pin it down.  Maybe doing division instead of multiplication will
 dodge the issue.
 
 Discussion: https://postgr.es/m/E1jkU7H-00024V-NZ@gemulon.postgresql.org