bus/driver: Only allow specific methods to be called at wrong paths

bus/driver: Only allow specific methods to be called at wrong paths

The default for the future should be that new functionality should
only be available at /org/freedesktop/DBus, which is the canonical
path of the "bus driver" object that represents the message bus.

The disallowed methods are still in Introspect() output, and
raise AccessDenied instead of UnknownMethod, to preserve the invariant
that the o.fd.DBus interface has constant contents.

test/dbus-daemon.c already asserts that UpdateActivationEnvironment()
still raises AccessDenied when invoked on a non-canonical path;
this has been in place since 1.8.14.

Reviewed-by: Philip Withnall <withnall@endlessm.com>
[smcv: Adjust comments, enum order, variable naming as per Philip's review]
Signed-off-by: Simon McVittie <smcv@collabora.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=101256