Move process launching into GDBRemoteCommunicationServer.

lldb-gdbserver was launching the commandline-specified launch process
directly, without GDBRemoteCommunicationServer knowing anything about
it.  As GDBRemoteCommunicationServer is the piece that manages and
knows about processes that the gdb remote protocol discusses with
the client end, it is important that it know about launched processes.

This change also implements the k gdb remote protocol message, having it
kill all known spawned processes when it is received.

(Note: in lldb-gdbserver, the spawned processes are not properly
monitored yet. The response to the k packet will complain that
spawned processes do not really appear to be getting killed even if
they are. This will get addressed soon.)