Do not send "new breakpoint" event when breakpoint is set

When the DAP client sets a breakpoint, gdb currently sends a "new
breakpoint" event.  However, Vladimir Makaev discovered that this
causes VSCode to think there are two breakpoints.

This patch changes gdb to suppress the event in this case.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30678