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 commit will change this. For this reason, tests are
introduced there.

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

This code is based on a patch from Jani Nikula.