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/vauth/cleartext.c

@@ -32,7 +32,6 @@
   (!defined(CURL_DISABLE_LDAP) && defined(USE_OPENLDAP))
 
 #include "vauth.h"
-#include "../sendf.h"
 
 /*
  * Curl_auth_create_plain_message()

lib/vauth/digest_sspi.c

@@ -32,7 +32,7 @@
 #include "vauth.h"
 #include "digest.h"
 #include "../curlx/multibyte.h"
-#include "../sendf.h"
+#include "../curl_trc.h"
 #include "../strdup.h"
 #include "../strcase.h"
 #include "../strerror.h"

lib/vauth/gsasl.c

@@ -29,7 +29,7 @@
 #ifdef USE_GSASL
 
 #include "vauth.h"
-#include "../sendf.h"
+#include "../curl_trc.h"
 
 #include <gsasl.h>
 

lib/vauth/krb5_gssapi.c

@@ -32,7 +32,7 @@
 #include "vauth.h"
 #include "../curl_sasl.h"
 #include "../curl_gssapi.h"
-#include "../sendf.h"
+#include "../curl_trc.h"
 
 #if defined(__GNUC__) && defined(__APPLE__)
 #pragma GCC diagnostic push

lib/vauth/krb5_sspi.c

@@ -29,7 +29,7 @@
 #if defined(USE_WINDOWS_SSPI) && defined(USE_KERBEROS5)
 
 #include "vauth.h"
-#include "../sendf.h"
+#include "../curl_trc.h"
 
 /*
  * Curl_auth_is_gssapi_supported()

lib/vauth/ntlm.c

@@ -36,7 +36,7 @@
 #define DEBUG_ME 0
 
 #include "vauth.h"
-#include "../sendf.h"
+#include "../curl_trc.h"
 #include "../curl_ntlm_core.h"
 #include "../rand.h"
 #include "../strdup.h"

lib/vauth/ntlm_sspi.c

@@ -28,7 +28,7 @@
 
 #include "vauth.h"
 #include "../curl_ntlm_core.h"
-#include "../sendf.h"
+#include "../curl_trc.h"
 #include "../strdup.h"
 
 /*

lib/vauth/spnego_gssapi.c

@@ -31,7 +31,7 @@
 #include "vauth.h"
 #include "../curlx/base64.h"
 #include "../curl_gssapi.h"
-#include "../sendf.h"
+#include "../curl_trc.h"
 
 #if defined(__GNUC__) && defined(__APPLE__)
 #pragma GCC diagnostic push

lib/vauth/spnego_sspi.c

@@ -30,7 +30,7 @@
 
 #include "vauth.h"
 #include "../curlx/base64.h"
-#include "../sendf.h"
+#include "../curl_trc.h"
 #include "../strerror.h"
 
 /*