detect xz/lzma support in rpm5

rpm5 doesn't use PayloadIsLzma, but rather has _lzma in "rpm --showrc".
Same with PayloadIsXz / _xz.

So we check for both with egrep.

The fix was tested with rpm(4) and rpm5 on Arch Linux,
where the "rpm" package is rpm5 and "rpm-org" is rpm(4).