move Windows unistd.h shim to its own directory

move Windows unistd.h shim to its own directory

Graphviz contains two shims for platforms lacking unistd.h:

  1. windows/include/unistd.h
  2. lib/ast/compat_unistd.h

(1) is more comprehensive than (2), and we would like to replace (2) entirely
with (1). However, it is not currently possible to put (1) in the compiler’s
include path during the CMake build. Because it lives in the same directory as
the config.h used in the MS Build build, putting it in the compiler’s include
path also makes this config.h eligible for inclusion. This conflicts with the
CMake build system’s own generated config.h.

Moving this unistd.h to its own directory allows us to more selectively put it
in the include path when relevant. Though surprisingly this file seems unused
right now; even the adjacent config.h does not define `HAVE_UNISTD_H`,
indicating it should not be included.

With unistd.h available natively on every supported platform except Windows,
this change should allow removing unistd.h detection in future and
unconditionally including it when desired.

Gitlab: #2204