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 250d613763 #15787
Follow-up to 5e70566094 #15169

Closes #20539

CMake/unix-cache.cmake

@@ -223,12 +223,6 @@ else()
 endif()
 set(HAVE_SENDMSG 1)
 set(HAVE_SETLOCALE 1)
-if(CYGWIN OR
-   CMAKE_SYSTEM_NAME STREQUAL "Linux")
-  set(HAVE_SETMODE 0)
-else()
-  set(HAVE_SETMODE 1)
-endif()
 set(HAVE_SETRLIMIT 1)
 set(HAVE_SETSOCKOPT_SO_NONBLOCK 0)
 set(HAVE_SIGACTION 1)
@@ -305,8 +299,5 @@ set(HAVE_UTIME 1)
 set(HAVE_UTIMES 1)
 set(HAVE_UTIME_H 1)
 set(HAVE_WRITABLE_ARGV 1)
-if(CYGWIN)
-  set(HAVE__SETMODE 1)
-endif()
 set(STDC_HEADERS 1)
 set(USE_UNIX_SOCKETS 1)