* child_info.h (child_info_fork::handle_failure): Declare new function.

* child_info.h (child_info_fork::handle_failure): Declare new function.
(child_info_fork::retry): New field.
* dcrt0.cc (__api_fatal_exit_val): Define.
(child_info_fork::handle_failure): Define new function.
(__api_fatal): Exit using __api_fatal_exit_val value.
* environ.cc (set_fork_retry): Set fork_retry based on CYGWIN environment
variable.
(parse_thing): Add "fork_retry" setting.
* fork.cc (fork_retry): Define.
(frok::parent): Reorganize to allow retry of failed child creation if child
signalled that it was ok to do so.
* heap.cc (heap_init): Signal parent via handle_failure when VirtualAlloc
fails.
* pinfo.h (EXITCODE_RETRY): Declare.
* sigproc.cc (child_info::sync): Properly exit with failure condition if called
for fork and didn't see subproc_ready.
* spawn.cc (spawn_guts): Use windows pid as first argument.
* winsup.h: Remove obsolete NEW_MACRO_VARARGS define.
(__api_fatal_exit_val): Declare.
(set_api_fatal_return): Define.
(in_dllentry): Declare.
* exceptions.cc (inside_kernel): Remove unneeded in_dllentry declaration.