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/socketpair.h

@@ -33,7 +33,6 @@
 #define wakeup_close         close
 #define wakeup_create(p, nb) Curl_eventfd(p, nb)
 
-#include <curl/curl.h>
 int Curl_eventfd(curl_socket_t socks[2], bool nonblocking);
 
 #elif defined(HAVE_PIPE)
@@ -43,7 +42,6 @@ int Curl_eventfd(curl_socket_t socks[2], bool nonblocking);
 #define wakeup_close         close
 #define wakeup_create(p, nb) Curl_pipe(p, nb)
 
-#include <curl/curl.h>
 int Curl_pipe(curl_socket_t socks[2], bool nonblocking);
 
 #else /* !USE_EVENTFD && !HAVE_PIPE */
@@ -73,8 +71,6 @@ int Curl_pipe(curl_socket_t socks[2], bool nonblocking);
 #endif /* USE_EVENTFD */
 
 #ifndef CURL_DISABLE_SOCKETPAIR
-#include <curl/curl.h>
-
 int Curl_socketpair(int domain, int type, int protocol,
                     curl_socket_t socks[2], bool nonblocking);
 #endif