When using `imap-shell-program' (with a nnimap-stream of 'shell), the
process is created `as is' with start-process.  The docstring of
start-process says :

        If you want to separate standard output from standard error,
        invoke the command through a shell and redirect one of them
        using the shell syntax.

Well, this is fun, because my distant imapd says the following :

$ imapd
* PREAUTH Ready.
INFO: LOGIN, user=cadilhac, ip=[127.0.0.1], protocol=IMAP

Oh, heck.  Gnus is searching for PREAUTH on the INFO line, doesn't found
it, and just wait until the end of time to see what's happening next[1].

Well, I'm not comfortable with the use of `2> /dev/null' on a package
meant to work on Windows, so I propose this more radical change :