This is needed to e.g. allow encoding maybe types (once we add
G_DBUS_CAPABILITY_FLAGS_MAYBE_TYPES) if, and only if, that capability
has been negotiated with the peer (via authentication).
Things compile and the test-suite passes. Still need to hook up
gio.symbols and docs. There are still a bunch of TODOs left in the
sources that needs to be addressed.
Signed-off-by: David Zeuthen <davidz@redhat.com>