g.SP refers to the frame the goroutine was in the last time it was
scheduled out. Instead of calling proc.(*Process).stacktrace directly
we should call proc.(*Process).GoroutineStacktrace that substitutes
fresh values retrieved from thread registers when necessary.
This bug leads to occasional problems with `next`.
Refactored variables.go to separate calculation of a variable's address from
reading its value. This change is useful to implement the 'set' command
as well as the evaluation of more complex expressions (in the future).
Some of the goroutines stored in runtime.allg are in the dead state and
should not be displayed. The state is determined by the 'g.atomicstatus'
member.
gopc is the instruction of the `go` command that spawned this goroutine.
What we really want (unless we can get the PC from the thread) is the
value of sched.pc which is the value of the PC at the time it was
parked.
