dbus-spec, dbus-protocol: add ALLOW_INTERACTIVE_AUTHORIZATION flag

dbus-spec, dbus-protocol: add ALLOW_INTERACTIVE_AUTHORIZATION flag

Heavily based on a patch from Lennart Poettering.

This is useful for authentication frameworks such as polkit, but this
flag is supposed to be generic, and not be bound to any implementation
of such a framework.

The dbus specification already clarifies that unknown flags must be
ignored, the reference implementation and the other implementations we
checked indeed ignore any new flags, hence we should be fine with
compatibility here.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=83449