bug#14271: 24.3; MAILHOST set in mail-source.el when using pop3.el

[Glenn Morris]

Katsumi Yamaoka wrote:

> Maybe setting MAILHOST is necessary for a user who uses a movemail
> program that is not that of pop3.el.  But it should be needed only
> when running that program.  So, I'd like to modify the pop3.el code
> so that it binds MAILHOST temporarily.  A patch for Emacs 24.3 is
> below.  Could others review it?  (I'm going to commit it only to
> the trunk.)

It might be simpler to let-bind process-environment?