mx_check_mailbox() would update the header flags, but was not
returning MUTT_FLAGS back to the index loop.  That meant a screen
redraw was needed to be notified of externally modified flags.

Change maildir_update_flags() to return 1 if the flags were actually
changed.  Change maildir_check_mailbox() and mh_check_mailbox() to
return MUTT_FLAGS when that happens.

Thanks to jcdenton and mike-burns for the original patch.