This is currently unused but might be useful in the future. For
example, it might be nice with a way to bypass the current queue of
outgoing messages - having a flag enumeration allows us to add a
G_DBUS_SEND_MESSAGE_FLAGS_BYPASS_QUEUE etc. etc.

This commit breaks ABI and API. Users of the (rarely used) API to send
messages will have to port to this new API.

Signed-off-by: David Zeuthen <davidz@redhat.com>