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

tests/server/sockfilt.c

@@ -102,12 +102,8 @@
 #endif
 
 #include <curlx.h> /* from the private lib dir */
-#include "inet_pton.h"
-#include "timediff.h"
 #include "warnless.h" /* for read() */
 
-#include "tool_binmode.h"
-
 /* include memdebug.h last */
 #include <memdebug.h>
 
@@ -1495,9 +1491,9 @@ static int test_sockfilt(int argc, char *argv[])
     return 2;
 #endif
 
-  CURL_SET_BINMODE(stdin);
-  CURL_SET_BINMODE(stdout);
-  CURL_SET_BINMODE(stderr);
+  CURLX_SET_BINMODE(stdin);
+  CURLX_SET_BINMODE(stdout);
+  CURLX_SET_BINMODE(stderr);
 
   install_signal_handlers(false);