* notmuch: add placeholder since it has native thread support
* maildir: Return false since it does not have native thread support

Signed-off-by: Kevin Kuehler <keur@xcf.berkeley.edu>

This changes the search flags for maildir and imap backends.

They now no longer use -t for searching all text. This seems to make
more sense as being the targeted recipient. I have similarly added Cc
for -c. The text search now resides under -a for all text.

There is a command and config option. The criteria are a list of the
sort criterion and each can be individually reversed.

This only includes support for sorting in the maildir backend currently.
The other backends are not supported in this patch.

This populates the directory info model properly when requested,
allowing the fields to be relied upon elsewhere.

This also sends the dirinfo when new messages come in.

Basic searching is supported with the following:
- read messages
- unread messages
- from addresses
- text in body
- text in subject
- text in all

The implementation loops through all messages in the selected directory.
It tries to be smart by detecting which parts of each message the search
query needs to use and only loads these from the filesystem.

Things like FetchEntityPartReader etc can be reused by most workers
working with raw email files from disk (or any reader for that matter).

This patch extract that common functionality in a separate package.

This removes old aborted deliveries from the tmp directory.

This allows backends which can't always be compiled due to missing
dependencies (say libnotmuch) to be compiled conditionally with buildflags.