Add new timeout functions to work with inotify monitors.

Add new timeout functions to work with inotify monitors.

The ncurses timeout() function doesn't affect the new poll inside
mutt_monitor_poll().  This meant that $imap_keepalive and $timeout
were not being respected when the monitor was used.

Create mutt_getch_timeout(), which delegates to timeout() and sets a
timeout value mutt_monitor_poll() uses too.