#1 TAR files without "ustar" signatures beep

[SAndreason]

tardiff uses a block of 5 bytes found at a static
location in each file header within a TAR archive to
identify the file header and ensure that misalignments
caused by errors reading the archive will be noticed
(via beeping). Unfortunately, it appears that there's
a TAR archive creator that does not include this
signature in the archives it creates. The question is,
what should tardiff do?

More information on the problem and a copy of a
discussion about possible solutions is available at
http://tardiff.sourceforge.net/discuss-200506.html

At present, when no header is found, tardiff beeps and
continues on. Depending on which archive is in
question, this results in an archive of everything in
the original or a blank (except for the inevitable
"delete everything" script).