Tests for hashes in scalar context (and hence HvFILL()).
Perl_hv_fill() can return early if the hash only has 0 or 1 keys.

No keys implies no chains used, so the return value is 0. One key
unambiguously means 1 chain used, and all the others are free. Two or more
keys might share the same chain, or might not, so the calculation can't be
short-circuited.
Remove DG/UX support.

DG/UX was a Unix sold by Data General. The last release was in April 2001.
It only runs on Data General's own hardware.