Add logic to MachThreadList::GetThreadID() for the use case of setting a watchpoint

(MachThreadList::EnableHardwareWatchpoint()) where the watchpoint is not associated
with a thread and the current thread, if set, is returned, otherwise we return the
first thread.

Plus minor change to RNBRemote::HandlePacket_z() to use the existing macros to check
the validity of break_id/watch_id.

llvm-svn: 139246