* cygheap.cc (dup_now): New function.

* cygheap.cc (dup_now): New function.
(cygheap_setup_for_child): Accept new argument controlling whether to delay
copying of cygheap to shared memory region.
(cygheap_setup_for_child_cleanup): Accept new arguments controlling whether to
copy cygheap at this point.
* cygheap.h: Reflect above changes.
* fork.cc (fork_parent): Break copying of cygheap into two parts when
fork_fixup is required so that the child can see the parent's changes.
(vfork): Do stack cleanup prior to forcing a fork error.
* spawn.cc (spawn_guts): Ditto.