Add $imap_poll_timeout to allow mailbox polling to time out.

Add $imap_poll_timeout to allow mailbox polling to time out.

Enable the polling flag for the NOOP in imap_check_mailbox(), the
STATUS command in imap_buffy_check(), and the LOGOUT command.

This is not intended to handle all blocking-IO related issues.
However, the periodic NOOP and STATUS are the most frequent places for
mutt to freeze up, especially after a laptop is sleep/woken.

Since these are quick operations with little data, this is a good place
to check if the connection is still working before hanging on a read.