notmuch search: Change default search order to be newest messages first.

notmuch search: Change default search order to be newest messages first.

This is what most people want for a _search_ command. It's often
different for actually reading mail in an inbox, (where it makes more
sense to have results displayed in chronological order), but in such a
case, ther user is likely using an interface that can simply pass the
--sort=oldest-first option to "notmuch search".

Here we're also change the sort enum from NOTMUCH_SORT_DATE and
NOTMUCH_SORT_DATE_REVERSE to NOTMUCH_SORT_OLDEST_FIRST and
NOTMUCH_SORT_NEWEST_FIRST. Similarly we replace the --reverse option
to "notmuch search" with two options: --sort=oldest-first and
--sort=newest-first.

Finally, these changes are all tracked in the emacs interface, (which
has no change in its behavior).