journal: add minimal journal gateway daemon based on GNU libmicrohttpd

journal: add minimal journal gateway daemon based on GNU libmicrohttpd

This minimal HTTP server can serve journal data via HTTP. Its primary
purpose is synchronization of journal data across the network. It serves
journal data in three formats:

       text/plain: the text format known from /var/log/messages
       application/json: the journal entries formatted as JSON
       application/vnd.fdo.journal: the binary export format of the journal

The HTTP server also serves a small HTML5 app that makes use of the JSON
serialization to present the journal data to the user.

Examples:

This downloads the journal in text format:

 # systemctl start systemd-journal-gatewayd.service
 # wget http://localhost:19531/entries

Same for JSON:

 # curl -H"Accept: application/json" http://localhost:19531/entries

Access via web browser:

 $ firefox http://localhost:19531/