Add new event and trace recording API

The main change in this release is the addition of a new API for event and trace recording. There are also a few documentation updates and bug fixes (release getaddrinfo data to fix memory leak).

The goal of the new API is to allow all applications in the House series to record historical information of interest to the end-user (events) or to the system administrator (traces) in similar ways and formats.

The events and traces are saved to files in /dev/shm, and then to permanent storage. The rationale for this two-stage approach is to avoid applications to write small amount of data to SD cards continuously, wearing them down (e.g. Raspberry Pi). Storing files to /dev/shm makes it possible to save the data between application restart, but still write to permanent storage only once in a while.

The houseportal application itself uses this API for its own recording, but please realize that there is no end-user to the houseportal application really, since the portal is meant to be transparent to the users. This makes houseportal not such a great example of the semantic difference between events and traces.