lib: make CURLX_SET_BINMODE() and use it

Use it from libtests' `first.c` and thus also from units, and tunits.

Also:
- cmake: drop stray `curltool` lib dependency for units.
- units: stop depending on `src` headers.
- tests/server: drop depending on `src` headers.
  (the remaining one listed in the comments, `tool_xattr.h`, was not
  actually used from servers.)
- tests/server: drop duplicate curlx headers.
  (Except `warnless.h`, which is tricky on Windows.)

Closes #17672

lib/Makefile.inc

@@ -39,6 +39,7 @@ LIB_CURLX_CFILES = \
   curlx/winapi.c
 
 LIB_CURLX_HFILES = \
+  curlx/binmode.h  \
   curlx/base64.h   \
   curlx/curlx.h    \
   curlx/dynbuf.h   \

lib/curlx/binmode.h

@@ -0,0 +1,39 @@
+#ifndef HEADER_CURL_TOOL_BINMODE_H
+#define HEADER_CURL_TOOL_BINMODE_H
+/***************************************************************************
+ *                                  _   _ ____  _
+ *  Project                     ___| | | |  _ \| |
+ *                             / __| | | | |_) | |
+ *                            | (__| |_| |  _ <| |___
+ *                             \___|\___/|_| \_\_____|
+ *
+ * Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+ *
+ * This software is licensed as described in the file COPYING, which
+ * you should have received as part of this distribution. The terms
+ * are also available at https://curl.se/docs/copyright.html.
+ *
+ * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+ * copies of the Software, and permit persons to whom the Software is
+ * furnished to do so, under the terms of the COPYING file.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ * SPDX-License-Identifier: curl
+ *
+ ***************************************************************************/
+#include "../curl_setup.h"
+
+#if (defined(HAVE_SETMODE) || defined(HAVE__SETMODE)) && defined(O_BINARY)
+/* Requires io.h and/or fcntl.h when available */
+#ifdef HAVE__SETMODE
+#  define CURLX_SET_BINMODE(stream)  (void)_setmode(fileno(stream), O_BINARY)
+#else
+#  define CURLX_SET_BINMODE(stream)  (void)setmode(fileno(stream), O_BINARY)
+#endif
+#else
+#  define CURLX_SET_BINMODE(stream)  (void)stream; Curl_nop_stmt
+#endif
+
+#endif /* HEADER_CURL_TOOL_BINMODE_H */

lib/curlx/curlx.h

@@ -31,6 +31,9 @@
  * be.
  */
 
+#include "binmode.h"
+/* "binmode.h" provides macro CURLX_SET_BINMODE() */
+
 #include "nonblock.h"
 /* "nonblock.h" provides curlx_nonblock() */