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

lib/CMakeLists.txt

@@ -40,12 +40,8 @@ list(APPEND HHEADERS ${CMAKE_CURRENT_BINARY_DIR}/curl_config.h)
 # The rest of the build
 
 include_directories(
-  ${CMAKE_CURRENT_BINARY_DIR}/../include
-  ${CMAKE_CURRENT_SOURCE_DIR}/..
-  ${CMAKE_CURRENT_SOURCE_DIR}/../include
-  ${CMAKE_CURRENT_BINARY_DIR}/..
-  ${CMAKE_CURRENT_SOURCE_DIR}
-  ${CMAKE_CURRENT_BINARY_DIR}
+  "${CMAKE_CURRENT_BINARY_DIR}"
+  "${CMAKE_CURRENT_SOURCE_DIR}"
 )
 if(USE_ARES)
   include_directories(${CARES_INCLUDE_DIR})