compatibility with future PGP

[hughes@ah.com (Eric Hughes)]

The only change the future post-September PGP 2.6 messages will have
is a change in the version number byte from 2 to 3.  PC's little hack
not to check version numbers will work, but as a patch it's not the
most robust.  It would be more robust if it checked for the range
[2..3].

Another thing a patched 2.3 release would have to do to be fully
indistinguishable is to generate new version numbers itself after the
given date.

Eric