Major update to the xzgrep and other scripts based on

Major update to the xzgrep and other scripts based on
the latest versions found from gzip CVS repository.

configure will try to find a POSIX shell to be used by
the scripts. This should ease portability on systems
which have pre-POSIX /bin/sh.

xzgrep and xzdiff support .xz, .lzma, .gz, and .bz2 files.
xzmore and xzless support only .xz and .lzma files.

The name of the xz executable used in these scripts is
now correct even if --program-transform-name has been used.