lib: drop, or replace sendf.h with curl_trc.h where possible

- replace `sendf.h` with `curl_trc.h` where it was included just for it.
- drop unused `curl_trc.h` includes.
- easy: delete obsolete comment about `send.h` include reason.

Also:
- move out `curl_trc.h` include from `sendf.h` and include it directly
  in users, where not done already. To flatten the include tree and
  to less rely on indirect includes.
- stop including `sendf.h` from other headers, replace it with forward
  declaration of `Curl_easy`, as done already elsewhere.

Verified with an all non-unity CI run.

Closes #20061

lib/vtls/gtls.c

@@ -41,7 +41,7 @@
 #include <nettle/sha2.h>
 
 #include "../urldata.h"
-#include "../sendf.h"
+#include "../curl_trc.h"
 #include "keylog.h"
 #include "gtls.h"
 #include "vtls.h"

lib/vtls/mbedtls.c

@@ -60,7 +60,7 @@
 #endif
 #include "cipher_suite.h"
 #include "../urldata.h"
-#include "../sendf.h"
+#include "../curl_trc.h"
 #include "mbedtls.h"
 #include "vtls.h"
 #include "vtls_int.h"

lib/vtls/openssl.c

@@ -44,7 +44,7 @@
 #endif
 
 #include "../urldata.h"
-#include "../sendf.h"
+#include "../curl_trc.h"
 #include "../formdata.h" /* for the boundary function */
 #include "../url.h" /* for the ssl config check function */
 #include "../curlx/inet_pton.h"

lib/vtls/rustls.c

@@ -33,7 +33,7 @@
 #include "../curlx/fopen.h"
 #include "../curlx/strerr.h"
 #include "../urldata.h"
-#include "../sendf.h"
+#include "../curl_trc.h"
 #include "vtls.h"
 #include "vtls_int.h"
 #include "rustls.h"

lib/vtls/schannel.c

@@ -42,7 +42,7 @@
 #include "vtls.h"
 #include "vtls_int.h"
 #include "vtls_scache.h"
-#include "../sendf.h"
+#include "../curl_trc.h"
 #include "../connect.h" /* for the connect timeout */
 #include "../strdup.h"
 #include "../strerror.h"

lib/vtls/schannel_verify.c

@@ -43,7 +43,7 @@
 #include "../curlx/inet_pton.h"
 #include "vtls.h"
 #include "vtls_int.h"
-#include "../sendf.h"
+#include "../curl_trc.h"
 #include "../strerror.h"
 #include "../curlx/winapi.h"
 #include "../curlx/multibyte.h"

lib/vtls/vtls.c

@@ -59,7 +59,7 @@
 #include "rustls.h"         /* Rustls versions */
 
 #include "../slist.h"
-#include "../sendf.h"
+#include "../curl_trc.h"
 #include "../strcase.h"
 #include "../url.h"
 #include "../progress.h"

lib/vtls/wolfssl.c

@@ -54,7 +54,7 @@
 #endif
 
 #include "../urldata.h"
-#include "../sendf.h"
+#include "../curl_trc.h"
 #include "vtls.h"
 #include "vtls_int.h"
 #include "vtls_scache.h"

lib/vtls/x509asn1.c

@@ -39,7 +39,7 @@
 
 #include "../urldata.h"
 #include "vtls.h"
-#include "../sendf.h"
+#include "../curl_trc.h"
 #include "../curlx/base64.h"
 #include "x509asn1.h"
 #include "../curlx/dynbuf.h"