unistd-safer: add unit test

unistd-safer: add unit test

Add more unit tests.  Meanwhile, fix compilation error on mingw when
testing unistd-safer and fchdir together; and avoid gcc warning on
platforms without setmode.

* modules/unistd-safer-tests: New file.
* tests/test-dup-safer.c: Likewise.
* tests/test-cloexec.c (setmode): Avoid compiler warning.
* tests/test-dup2.c (setmode): Likewise.
* lib/cloexec.c (dup_cloexec): Fix mingw compile error.

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