Implement DAP 'terminated' event

This implements the DAP 'terminated' event.  Vladimir Makaev noticed
that VSCode will not report the debug session as over unless this is
sent.

It's not completely clear when exactly this event ought to be sent.
Here I've done it when the inferior exits.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30681



3 files changed