(pipe2): Export.
* dtable.cc (dtable::dup_worker): Take additional flags parameter.
Handle O_CLOEXEC flag.
(dtable::dup3): Rename from dup2. Take additional flags parameter.
Check for valid flags. Drop check for newfd == oldfd.
* dtable.h (dtable::dup_worker): Add flags parameter.
(dtable::dup3): Rename from dup2.
* fcntl.cc (fcntl64): Add F_DUPFD_CLOEXEC case.
* fhandler.h (fhandler_mailslot::get_object_attr): Add flags parameter.
* fhandler.cc (fhandler_base::open): Use security attribute with
inheritance according to setting of O_CLOEXEC flag.
* fhandler_console.cc (fhandler_console::open): Ditto.
* fhandler_fifo.cc (sec_user_cloexec): New inline function to
create security attribute with inheritance according to setting of
O_CLOEXEC flag.
(fhandler_fifo::open): Call sec_user_cloexec to fetch security
attribute.
(fhandler_fifo::wait): Ditto.
* fhandler_mem.cc (fhandler_dev_mem::open): Ditto.
* fhandler_mailslot.cc (fhandler_mailslot::get_object_attr): Take
additional flags parameter. Use security attribute with inheritance
according to setting of O_CLOEXEC flag.
(fhandler_mailslot::open): Call get_object_attr with flags parameter.
* fhandler_registry.cc (fhandler_registry::open): Call set_close_on_exec
on real handles to accommodate O_CLOEXEC flag.
* fhandler_tty.cc (fhandler_tty_slave::open): Ditto.
* fhandler_tape.cc: Create mutex with inheritance according to setting
of O_CLOEXEC flag.
* pipe.cc: Replace usage of O_NOINHERIT with O_CLOEXEC.
(fhandler_pipe::init): Simplify setting close_on_exec flag.
(fhandler_pipe::open): Remove setting close_on_exec flag.
(fhandler_pipe::create): Use security attribute with inheritance
according to setting of O_CLOEXEC flag.
(pipe2): New exported function.
* posix_ipc.cc: Throughout, open backing files with O_CLOEXEC
flag to follow POSIX semantics.
* security.h (sec_none_cloexec): New define.
* syscalls.cc (dup): Add missing extern "C" qualifier. Accommodate
renaming of dtable::dup2 to dtable::dup3.
(dup2): Ditto. Check newfd == oldfd here.
(dup3): New function. Check newfd == oldfd here.
(open): Set close_on_exec flag according to O_CLOEXEC flag before
calling fhandler->open.
* include/cygwin/version.h (CYGWIN_VERSION_API_MINOR): Bump.
e70fdfb99f