[dbus] Enablers for limiting dbus methods to privileged clients. JB#3…

…8607

D-Bus configuration mechanisms do not support allowing/denying method calls
based on effective uid/gid. While mce already has multiple partially
overlapping ways to track clients, none of these are suitable for effective
uid/gid checking.

Track all D-Bus peers doing ipc with mce and maintain a cache of properties
relevant for privilege checks for each peer.

Delay execution of privileged method calls until the peer details are known
so that access can be granted / denied.

Rewrite existing client exit, service availability and debug identity
tracking to utilize the same logic that is used for privilege checks.

Also remove / hide some mce-dbus functions that are not used by other
parts of mce code base.

Signed-off-by: Simo Piiroinen <simo.piiroinen@jollamobile.com>