* fork.cc (vfork): Store complete stack frame in vfork_save structure for later

* fork.cc (vfork): Store complete stack frame in vfork_save structure for later
recovery.
* spawn.cc (spawn_guts): Reorganize slightly to consolidate handling when there
is a CreateProcess error.
(_spawnve): Only longjmp back to vfork handling when a process has been
successfuly started.
* winsup.h (vfork_save): Extend to include frame info.  Remove obsolete cpplus
conditionals.