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: #15652Closes#15787
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