Handle StackFrameFormat in DAP

Handle StackFrameFormat in DAP

DAP specifies a StackFrameFormat object that can be used to change how
the "name" part of a stack frame is constructed.  While this output
can already be done in a nicer way (and also letting the client choose
the formatting), nevertheless it is in the spec, so I figured I'd
implement it.

While implementing this, I discovered that the current code does not
correctly preserve frame IDs across requests.  I rewrote frame
iteration to preserve this, and it turned out to be simpler to combine
these patches.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30475