Patch motivation:

Bash  has a mail checking feature that, by default, checks your mail
spool file every 60 seconds (or 600 if you have POSIXLY_CORRECT
set).  You can disable this feature by unsetting your MAILCHECK
environment variable or setting it to a negative value.
However, even if you do this, bash will still check the timestamp and
file size of your mail file when it first starts in interactive mode
as well as when any of the MAIL, MAILCHECK, and MAILPATH environment
changes value.  The end result is that every time you start
an interactive bash shell, your mail file is checked.  For systems
where the mail spool directory is mounted via NFS or some other
network file system, this behavior can become a performance drag on
both the client and server.


If you're a wireless laptop user, you don't want to wait for the
automounter to mount the mail spool every time you open a shell
window. Worse, if the NFS server goes down, you may not be able to log
in to a client, even with a local account, because bash will hang on
an I/O operation while trying to check your mail.  Therefore, it is
desirable to make bash not stat the mail file at all when MAILCHECK
is unset MAILCHECK or set it to a negative value.  Bash shouldn't
examine our mail file at all unless we change the value of
MAILCHECK back to a non-negative value.

This patch implements the above behavior without affecting the normal
behavior for non-negative values of MAILCHECK.  Someone more intimate
with the bash architecture may have a prettier way to implement the
change.