Removed ProcessMessage from NativeProcessLinux.

Partial implementation for:
https://github.com/tfiala/lldb/issues/23

It's not clear to me yet whether the stub needs to stop all threads
on the following conditions, as the local debugging does:

1. breakpoint/trace hit.
2. watchpoint hit.
3. crash.
4. new thread.

If I need to stop-all on all of those, I have a plan but it will
require some changes.  My hope is that NativeProcessProtocol doesn't
do that and rather it is something around it that handles it.
-