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/mprintf.c

@@ -23,6 +23,7 @@
  */
 
 #include "curl_setup.h"
+
 #include "curlx/dynbuf.h"
 #include "curl_printf.h"
 #include "curlx/strparse.h"