cmake: cleanup header paths

- sync build-dir/source-dir header path order with autotools, by
  including build-dir first, then source-dir.
  This prevents out-of-tree builds breaking due to leftover generated
  headers in the source tree.

- tests/unit: move `src` ahead of `libtest` in header path, syncing with
  autotools.

- stop adding non-existing generated `include` dir to header path.
  There are no generated `include` headers and this directory is either
  missing in out-of-tree builds or the same as the one already added
  globally via the root `CMakeLists.txt`.

- lib: stop adding a duplicate source include directory to the header
  path.
  It's already added globally via the root `CMakeLists.txt`.

- lib: stop adding the project root to the header path.

- docs/examples: drop internal header paths.
  Examples do not and should not use internal headers.

- replace `curl_setup_once.h` in comments with `curl_setup.h`,
  the header actually used, and also referred to in autotools comments.

- add comment why we need `src` in include path for `tests/server`.

- add quotes around header directories.

Closes #14416

src/CMakeLists.txt

@@ -118,11 +118,10 @@ source_group("curl source files" FILES ${CURL_CFILES})
 source_group("curl header files" FILES ${CURL_HFILES})
 
 include_directories(
-  ${CURL_SOURCE_DIR}/lib      # for "curl_setup_once.h"
-  ${CURL_BINARY_DIR}/lib      # for "curl_config.h"
-  ${CURL_BINARY_DIR}/include  # for "curl/curl.h"
+  "${CURL_BINARY_DIR}/lib"  # for "curl_config.h"
+  "${CURL_SOURCE_DIR}/lib"  # for "curl_setup.h"
   # This is needed as tool_hugehelp.c is generated in the binary dir
-  ${CURL_SOURCE_DIR}/src      # for "tool_hugehelp.h"
+  "${CURL_SOURCE_DIR}/src"  # for "tool_hugehelp.h"
 )
 
 # Build curl executable