xzless: Make "less -V" parsing more robust

xzless: Make "less -V" parsing more robust

In v4.999.9beta~30 (xzless: Support compressed standard input,
2009-08-09), xzless learned to parse ‘less -V’ output to figure out
whether less is new enough to handle $LESSOPEN settings starting
with “|-”.  That worked well for a while, but the version string from
‘less’ versions 448 (June, 2012) is misparsed, producing a warning:

	$ xzless /tmp/test.xz; echo $?
	/usr/bin/xzless: line 49: test: 456 (GNU regular expressions): \
	integer expression expected
	0

More precisely, modern ‘less’ lists the regexp implementation along
with its version number, and xzless passes the entire version number
with attached parenthetical phrase as a number to "test $a -gt $b",
producing the above confusing message.

	$ less-444 -V | head -1
	less 444
	$ less -V | head -1
	less 456 (no regular expressions)

So relax the pattern matched --- instead of expecting "less <number>",
look for a line of the form "less <number>[ (extra parenthetical)]".
While at it, improve the behavior when no matching line is found ---
instead of producing a cryptic message, we can fall back on a LESSPIPE
setting that is supported by all versions of ‘less’.

The implementation uses "awk" for simplicity.  Hopefully that’s
portable enough.

Reported-by: Jörg-Volker Peetz <jvpeetz@web.de>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>