Extension: On-demand API to add peer credentials on every message

[zeenix]

@poettering needs this in systemd.

[poettering]

Yeah. I want a dbus "driver" call a client can make. Should take "as", an array of strings indicating the type of credentials the client wants to have appended implicitly to all method call messages that it gets delivered to it.

Benefit: no more round trips for authenticating client calls (well, pk will still be necessary)

[poettering]

same issue requested from dbus-broker btw:

bus1/dbus-broker#311