Provides functions for implementing finer and dynamic D-Bus access
control that supplements the built-in D-Bus security mechanisms.
The functions fall into these two groups:

* Checking the peer's credentials
* Parsing and applying access control policies

The peer's credentials are queried with GetConnectionUnixProcessID
and the results are cached for certain period of time or until the
peer disappears.

It allows to get around the following issues and limitations of the
built-in D-Bus security policies:

* They ignore the effective group id of the caller
* They don't analyze the arguments of the call
* They don't allow to select actions based on credentials
* They are static