gzip: port zdiff, zless to Busybox

Problem reported by Denys Zagorui (Bug#26088).
* tests/zdiff: Check that diff uses POSIX-format output.
* zless.in (less_version): Don't exit merely because 'less -V'
fails; instead, assume 'less' is compatible with an old version of
the original 'less'.  Busybox 'less -V' fails, but apparently its
'less' works anyway somehow.