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/Makefile.am

@@ -31,14 +31,12 @@ AUTOMAKE_OPTIONS = foreign nostdinc
 # $(top_srcdir)/include is for libcurl's external include files
 # $(top_builddir)/lib is for libcurl's generated lib/curl_config.h file
 # $(top_srcdir)/lib for libcurl's lib/curl_setup.h and other "borrowed" files
-# $(top_srcdir)/src for "tool_binmode.h", "tool_xattr.h"
 # $(srcdir) for the generated bundle source to find included test sources
 
 AM_CPPFLAGS = -I$(top_srcdir)/include        \
               -I$(top_builddir)/lib          \
               -I$(top_srcdir)/lib            \
               -I$(top_srcdir)/lib/curlx      \
-              -I$(top_srcdir)/src            \
               -I$(srcdir)
 
 # Get BUNDLE, BUNDLE_SRC, FIRSTFILES, UTILS, CURLX_SRCS, TESTFILES variables