zless: improve gzip failure checking, and port to new -V format

Problem reported by Jaroslaw Weglinski, and LESSOPEN change
suggested by Mark Nudelman, in: http://bugs.gnu.org/16951
This doesn't fix bug 16951 entirely, as 'less' needs to be changed
too, but it's a start.
* zless.in (check_exit_status): New var.
(LESSOPEN): Use it.
(use_input_pipe_on_stdin): Adjust to output format on Fedora 20,
where 'less -V' outputs "less 458 (POSIX regular expressions)"
on the first line.