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   \