Error if multiprocess not supported

[tromey]

I think the current plan is to only support the multiprocess extensions. So, we should error if the client does not send the multiprocess feature in the initial feature query packet.

[tromey]

We can't do this, since lldb doesn't support this extension. I think it's fine to continue parsing multiprocess thread-ids unconditionally, but we should be more careful with the responses we send. Some investigation is needed to see how lldb represents threads - like maybe it sends the TID as the PID?

[tromey]

See https://github.com/tromey/rust-gdb-remote-protocol/tree/client-state

However it's up to the library user to know whether multiple processes are supported, and that branch doesn't yet expose the information. So, a bit more work has to be done.