binmode: convert to macro and use it from tests

binmode: convert to macro and use it from tests

And use it from src and tests.

Syncing this functionality between platforms and build targets.

Also: Stop redefining `O_BINARY` in src, and use a local macro with
the same effect. `O_BINARY` is used in `CURL_SET_BINMODE()` to decide
if this functionality is supported, and redefining it makes this check
pass always in unity builds. The check is required for Apple OS, because
it offers a `setmode()` function, successfully detected by both CMake
and autotools, but that function has a different functionality and
signature than that expected by `CURL_SET_BINMODE()`.

Also:
- drop MetaWare High C (MS-DOS) support for set binmode.
- tests/libtest/Makefile.inc: dedupe comments.
- lib/curl_setup_once.h: tidy up feature guards for `io.h`, `fcntl.h`.

Ref: #15652
Closes #15787