Overlay, OverlayClient: add PID and process name diagnostics to 'Dead client detected' notice.

I'm going to use this to diagnose some regular client freezes I've been experiencing on Windows.

My own client is running with this patched in as we speak.  I think the culprit of the freezes/hangs are Spotify, but I'll need to run with the patch some more to be sure.

I think these extra diagnostics make sense to have upstream as well.  Thoughts?
If you're referring to the name, I think our policy is to only use the new guidelines for new files?

I think having a m_processName in here would be weird.