* Added support for authentication codes for the VM service.

Previously, a valid web socket connection would use the following URI:

`ws://127.0.0.1/ws`

Now, by default, the VM service requires a connection to be made with a
URI similar to the following:

`ws://127.0.0.1:8181/Ug_U0QVsqFs=/ws`

where `Ug_U0QVsqFs` is an authentication code generated and shared by
the
service.

This behavior can be disabled with the `--disable-service-auth-codes`
flag.