merge: upstream fixes (mutt/default)

 * Fix menu color calls to occur before positioning the cursor. (see #3956)
 * Change imap_cmd_start() to return OK when the cmd_queue is finished. (closes #3956)
 * When guessing an attachment type, don't allow text/plain if there is a null character. (see #2933)
 * Add timeout parameter to mutt_socket_poll.
 * Add $imap_poll_timeout to allow mailbox polling to time out.
 * Convert raw_socket_poll() to use gettimeofday().