Upgrade tar-snapshot-edit script.

* scripts/tar-snapshot-edit: Update Perl syntax to work
correctly with more recent versions of Perl.  (The original
code worked with in the v5.8 timeframe but not with
Perl v5.10.1 and later.)
Add a "-c" option to check the snapshot file for invalid
field values.
Handle NFS indicator character ("+") in version 0 and 1 files.
Preserve the original header/version line when editing version 1
or 2 files.
Tweak output formatting.
* doc/tar-snapshot-edit.texi: Update documentation.