shell mail checking

[utzoo!henry]

Bill Reeves of Lucasfilm has found a subtle bug in the Bourne shell's
mail checking:  if the mailbox is temporarily nonexistent (some mail
systems remove an empty mailbox), the shell can get a garbage mail date
into its memory, which can effectively suppress mail checking thenceforth.
The fix is like this, in main.c (line numbers are approximate):

139,140c139,141
< 			    ANDF mailtime
< 			THEN	prs(mailmsg)
---
> 			THEN
> 				IF mailtime THEN prs(mailmsg) FI
> 				mailtime=statb.st_mtime;
142d142
< 			mailtime=statb.st_mtime;