Implement the notStopped DAP response

Implement the notStopped DAP response

DAP specifies that a request can fail with the "notStopped" message if
the inferior is running but the request requires that it first be
stopped.

This patch implements this for gdb.  Most requests are assumed to
require a stopped inferior, and the exceptions are noted by a new
'request' parameter.

You may notice that the implementation is a bit racy.  I think this is
inherent -- unless the client waits for a stop event before sending a
request, the request may be processed at any time relative to a stop.

https://sourceware.org/bugzilla/show_bug.cgi?id=31037

Reviewed-by: Kévin Le Gouguec <legouguec@adacore.com>