Fix DAP launch and configurationDone requests

Fix DAP launch and configurationDone requests

Co-workers at AdaCore pointed out that gdb incorrectly implements the
DAP launch and configurationDone requests.  It's somewhat strange to
me, but the spec does in fact say that configuration requests should
occur before the executable is known to gdb.  This was clarified in
this bug report against the spec:

    https://github.com/microsoft/debug-adapter-protocol/issues/452

Fixing 'launch' to start the inferior was straightforward, but this
then required some changes to how breakpoints are handled.  In
particular, now gdb will emit the "pending" reason on a breakpoint,
and will suppress breakpoint events during breakpoint setting.