Add message bouncing capability

Add message bouncing capability

Bouncing a message is akin to redirecting a mail with a .forward
entry.  It is passed back to the mail system as it sits on disk.

By pressing ! while viewing a message, you can now re-inject it to the
mail system using either the command defined in bounce_sendmail or the
sendmail command for the default account with any instance of -t
removed. The user is prompted for the recipients of the message and is
offered a chance to confirm the bounce before it is sent.

The message is _not_ stored in the sent box, as this doesn't make
sense with bounced messages (and would not show up uniquely anyway).

The bounce_sendmail configuration item allows users that require
different sendmail commands depending on where the bounce is destined
to write a wrapper around their local mail tools to pick and choose
the appropriate injection method for the message based on the
addresses passed in.  Most systems can likely use: sendmail -oem -i

Signed-off-by: Ben Walton <bwalton@artsci.utoronto.ca>