* cygheap.h (init_cygheap): Play more vfork shell games and move ctty_on_hold

* cygheap.h (init_cygheap): Play more vfork shell games and move ctty_on_hold
and open_fhs_on_hold (back) here.
* dcrt0.cc (_dll_crt0): Just set impure_ptr_ptr here and let later
initialization deal with tls.
* dtable.cc (dtable::vfork_child_fixup): Move ctty considerations here.
(dtable:vfork_parent_restore): And here.
* fork.cc (vfork): Reflect change to ctty handling.
* perthread.h (vfork_save::fhctty): Eliminate.
* cygwin.din: Make more exports NOSIGFE that will never be interrupted by a
signal.
* init.cc (dll_entry): Set stackptr to NULL to catch problems earlier.