Per discussion on cygwin-developers, a Cygwin tmpfile(3) implementation
has been added to syscalls.cc. This overrides the one supplied by
newlib. Then the open(2) flag O_TMPFILE was added to the open call that
tmpfile internally makes.
This v2 patch removes O_CREAT from open() call as O_TMPFILE obviates it.
Note that open() takes a directory's path but returns an fd to a file.