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/unit/CMakeLists.txt

@@ -35,12 +35,11 @@ add_custom_command(OUTPUT "${BUNDLE_SRC}"
 
 add_executable(${BUNDLE} EXCLUDE_FROM_ALL ${UTILS} "${BUNDLE_SRC}")
 add_dependencies(testdeps ${BUNDLE})
-target_link_libraries(${BUNDLE} curltool curlu)
+target_link_libraries(${BUNDLE} curlu)
 target_include_directories(${BUNDLE} PRIVATE
   "${PROJECT_BINARY_DIR}/lib"            # for "curl_config.h"
   "${PROJECT_SOURCE_DIR}/lib"            # for "curl_setup.h"
   "${PROJECT_SOURCE_DIR}/lib/curlx"      # for curlx
-  "${PROJECT_SOURCE_DIR}/src"            # for "tool_binmode.h"
   "${PROJECT_SOURCE_DIR}/tests/libtest"  # for "first.h"
   "${CMAKE_CURRENT_SOURCE_DIR}"          # for the generated bundle source to find included test sources
 )

tests/unit/Makefile.am

@@ -37,7 +37,6 @@ AM_CPPFLAGS = -I$(top_srcdir)/include        \
               -I$(top_builddir)/lib          \
               -I$(top_srcdir)/lib            \
               -I$(top_srcdir)/lib/curlx      \
-              -I$(top_srcdir)/src            \
               -I$(top_srcdir)/tests/libtest  \
               -I$(srcdir)