* proc,proc/*: move SelectedGoroutine to proc.Target, remove PostInitializationSetup

moves SelectedGoroutine, SwitchThread and SwitchGoroutine to
proc.Target, merges PostInitializationSetup with NewTarget.

* proc,proc/*: add StopReason field to Target

Adds a StopReason field to the Target object describing why the target
process is currently stopped. This will be useful for the DAP server
(which needs to report this reason in one of its requests) as well as
making pull request #1785 (reverse step) conformant to the new
architecture.

* proc: collect NewTarget arguments into a struct