large mail messages

[utzoo!henry]

The V7 mail program screws up badly if a mail message (including postmark)
is larger than 32768 characters:  it loses the mail and appends a couple
of bytes of garbage to the mailbox.  Such large mail messages seldom occur
in intrasystem traffic but are not unheard-of when mailing files across
the network.  The problem is that the byte counter in copylet() is an int.
The fix is trivial:  in about line 329, change the declaration of k from
int to long.