build: tidy up and simplify `setmode()` detection and use

build: tidy up and simplify `setmode()` detection and use

- move macro to `curl_setup.h` (from curlx), and rename.
  It's required by src, test servers, libtests. Also used by unit/tunit,
  (which is fixable but this patch doesn't touch it.)
- special-case it for Windows/Cygwin/MS-DOS.
- build: drop `setmode()`/`_setmode()` detection.
  This also avoids detecting the different `setmode()` on BSDs,
  and a lot of complexity and overhead.
- use `CURL_O_BINARY`.

Follow-up to 250d613763dfc29f73010696ee7948f19d07dba9 #15787
Follow-up to 5e70566094463b8463d4dc1bec9b66763a761c76 #15169

Closes #20539