The attached patch adds to the debugger a capability I thought about

The attached patch adds to the debugger a capability I thought about
ages ago and which turned out to be absurdly easy to implement.  It adds
an optional parameter to the t(race) command, a maximum number of stack
frames to trace below the current one:, e.g.:

     t 3 - turn tracing on, trace up to 3 levels below current depth, be
silent below that
     t 2 fnord() - trace up to 2 levels deep in execution of fnord()

Since it is backwards compatible I added it to the legacy command set as
well, but that's certainly debatable.