build: drop duplicate include curl/curl.h and others

- curl_range: replace `sendf.h` with direct header dependency
  `curl_trc.h`.
- drop `curl/curl.h` includes from internal sourcees in favor of the
  include made from `curl_setup.h`. Replace it with the latter where
  it's the only include.
- include `curl_setup.h` before using macros, where missing.
- drop redundant `stdlib.h`, `string.h` includes, in favor of
  `curl_setup_once.h` including them.
- drop redundant `limits.h` in favor of `curl_setup.h` including it.
- fake_addrinfo.h: fix typo in comment.
- curl_setup_once.h: drop `stdio.h` in favor of earlier include in
  `curl_setup.h`.
- drop stray, unused, `stddef.h` includes.
- memdebug.h: add missing `stddef.h` include. (relying on accidental
  includes via other headers before this patch.)
- stddef.h: document why it's included.
- strerr: drop `curl/mprintf.h` in favor of `curl/curl.h` including it
  via `curl_setup.h`.

Closes #20027

lib/http2.c

@@ -27,6 +27,7 @@
 #if !defined(CURL_DISABLE_HTTP) && defined(USE_NGHTTP2)
 #include <stdint.h>
 #include <nghttp2/nghttp2.h>
+
 #include "urldata.h"
 #include "bufq.h"
 #include "uint-hash.h"
@@ -3045,7 +3046,6 @@ void *Curl_nghttp2_realloc(void *ptr, size_t size, void *user_data)
 #else /* CURL_DISABLE_HTTP || !USE_NGHTTP2 */
 
 /* Satisfy external references even if http2 is not compiled in. */
-#include <curl/curl.h>
 
 char *curl_pushheader_bynum(struct curl_pushheaders *h, size_t num)
 {