Enable the idata->check_status using bit operations

Enable the idata->check_status using bit operations

Commit e3f66d7e fixed dropped new mail notications, removing the
unsetting of idata->reopen IMAP_NEWMAIL_PENDING in imap_cmd_finish()
when an EXPUNGE was processed.

However, imap_cmd_finish() can be called twice by
imap_check_mailbox().  First as part of the imap_exec(), and manually
again just below.

Now that the IMAP_NEWMAIL_PENDING still exists, a second call could
overwrite idata->check_status if both reopen flags were set.

This unfortunately affects update_index(), which behaves differently
for MUTT_REOPENED.

I need to change the return value of mx_check_mailbox() in master to
preserve all the bits, so the index can both notify of new mail and
update_index() properly.

For stable, the best fix is to use bit operators to enable the
check_status flags in imap_cmd_finish() (and cmd_parse_fetch for
flags), and keep the imap_check_mailbox() priority of setting its
return value (it prioritizes IMAP_EXCHANGE_PENDING).

Co-authored-by: Richard Russon <rich@flatcap.org>