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

src/Makefile.inc

@@ -45,6 +45,7 @@ CURLX_CFILES = \
   ../lib/curlx/warnless.c
 
 CURLX_HFILES = \
+  ../lib/curlx/binmode.h \
   ../lib/curlx/multibyte.h \
   ../lib/curl_setup.h \
   ../lib/curlx/dynbuf.h \
@@ -105,7 +106,6 @@ CURL_HFILES = \
   config2setopts.h \
   slist_wc.h \
   terminal.h \
-  tool_binmode.h \
   tool_bname.h \
   tool_cb_dbg.h \
   tool_cb_hdr.h \