x/debug: read stack frames of goroutines.

Add some more fields and a String method to program.Frame, so they can
be printed similarly to how the runtime prints them.

Move stack-walking code to a separate function.

Change-Id: I3bc8a24296890733d13823a58b7eabd42050ca02
Reviewed-on: https://go-review.googlesource.com/19408
Reviewed-by: Dave Day <djd@golang.org>