xzless: Support compressed standard input

xzless: Support compressed standard input

It can be somewhat confusing that

	less < some_file.txt

works fine, whereas

	xzless < some_file.txt.xz

does not.  Since version 429, ‘less’ allows a filter specified in
the LESSOPEN environment variable to preprocess its input even if
it comes from standard input, if $LESSOPEN begins with ‘|-’.  So
set $LESSOPEN to take advantage of this feature.

Check less’s version at runtime so xzless can continue to work
with older versions.