Implement DAP logging breakpoints

Implement DAP logging breakpoints

DAP allows a source breakpoint to specify a log message.  When this is
done, the breakpoint acts more like gdb's dprintf: it logs a message
but does not cause a stop.

I looked into implement this using dprintf with the new %V printf
format.  However, my initial attempt at this did not work, because
when the inferior is continued, the dprintf output is captured by the
gdb.execute call.  Maybe this could be fixed by having all
inferior-continuation commands use the "&" form; the main benefit of
this would be that expressions are only parsed a single time.