Use non-deprecated CRT function variants on Windows.
- introduce `curlx_fdopen()`, `curlx_close()` and use them. Map them to
non-deprecated, underscored, CRT functions on Windows.
- replace `close()` uses with either `sclose()` (for sockets) or
`curlx_close()` (for files).
- map `fileno`, `unlink`, `isatty` to their non-deprecated, underscored,
versions on Windows.
- tool_dirhie: map `mkdir` to `_mkdir` on Windows.
- easy: use `_strdup()` on Windows, regardless of how `HAVE_STRDUP` is
set.
- cmake: assume `HAVE_STRDUP` on Windows. To allow dropping a detection
hack using `_CRT_NONSTDC_NO_DEPRECATE` with MSVC. Windows always has
`_strdup()` which the code uses, but also needs `HAVE_STRDUP` defined
to disable curl's own `strdup()` implementation.
- curl_setup.h: drop `_CRT_NONSTDC_NO_DEPRECATE` as no longer necessary.
Closes#20212
e50aa46fb2