cloexec: add dup_cloexec

cloexec: add dup_cloexec

This is needed to enforce correct semantics of mkostemp_safer.
Meanwhile, it is one step closer to providing O_CLOEXEC support
to open, as well as implementing portions of fcntl for mingw.

* lib/cloexec.h (dup_cloexec): New prototype.  Add copyright
header and comments.
* lib/cloexec.c (set_cloexec_flag): Add comments.
(dup_cloexec): New function, with mingw implementation borrowed
from...
* lib/w32spawn.h (dup_noinherit): ...here.
* modules/execute (Depends-on): Add cloexec.
* modules/pipe (Depends-on): Likewise.
* modules/cloexec (Depends-on): Add dup2.
* modules/cloexec-tests (Files): New file.
* tests/test-cloexec.c: Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>