* configure.in: Remove NEWVFORK default.

* configure.in: Remove NEWVFORK default.
* configure: Regenerate.
* dcrt0.cc: Conditionalize vfork stuff throughout.
* dtable.cc: Ditto.
* perthread.h: Ditto.
* pipe.cc (fhandler_pipe::close): Ditto.
* spawn.cc (spawnve): Ditto.
* syscalls.cc (setsid): Ditto.
* exceptions.cc (sigpacket::process): Use macro to refer to vfork pid.
* debug.cc (verify_handle): Define new function.
* debug.h (VerifyHandle): Define new macro.
(verify_handle): Declare new function
* fhandler.cc (fhandler_base::dup): Verify that dup'ed handle is not supposed
to be in use.
(fhandler_base::set_inheritance): Ditto.
(fhandler_base::fork_fixup): Ditto.
* fhandler_socket.cc (fhandler_socket::dup): Ditto.
* fhandler_tty.cc (fhandler_tty_slave::open): Ditto.
* net.cc (set_socket_inheritance): Ditto.
* pinfo.cc (pinfo_fixup_after_exec): Ditto.
* sigproc.cc (proc_subproc): Ditto.
(sig_send): Ditto.
* spawn.cc (spawn_guts): Ditto.
* thread.cc (pthread::init_mainthread): Ditto.
* pipe.cc (fhandler_pipe::close): Close read_state with ForceCloseHandle since
it was protected.
(fhandler_pipe::fixup_after_exec): Protect read_state handle.
(fhandler_pipe::dup): Correctly close open handles on error condition.  Verify
that dup'ed handle is not supposed to be in use.
(fhandler_pipe::create): Protect read_state.