cli: search: Add --output={sender,recipients}

cli: search: Add --output={sender,recipients}

The new outputs allow printing senders, recipients or both of matching
messages. To print both, the user can use --output=sender and
--output=recipients simultaneously.

Currently, the same address can appear multiple times in the output. The
next patch will change this. For this reason, the test are introduced in
the next patch as well.

We use mailbox_t rather than InternetAddressMailbox because we will need
to extend it in a following patch.

This code based on a patch from Jani Nikula.