Avoid exception from attach in DAP

Avoid exception from attach in DAP

I noticed that the DAP attach test case (and similarly
remoted-dap.exp) had a rogue exception stack trace in the log.  It
turns out that an attach will generate a stop that does not have a
reason.

This patch fixes the problem in the _on_stop event listener by making
it a bit more careful when examining the event reason.  It also adds
some machinery so that attach stops can be suppressed, which I think
is the right thing to do.

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