http, vauth: always provide Curl_allow_auth_to_host() functionality

This function is currently located in the lib/http.c module and is
therefore disabled by the CURL_DISABLE_HTTP conditional token.

As it may be called by TLS backends, disabling HTTP results in an
undefined reference error at link time.

Move this function to vauth/vauth.c to always provide it and rename it
as Curl_auth_allowed_to_host() to respect the vauth module naming
convention.

Closes #9600

lib/vauth/vauth.c

@@ -27,6 +27,8 @@
 #include <curl/curl.h>
 
 #include "vauth.h"
+#include "urldata.h"
+#include "strcase.h"
 #include "curl_multibyte.h"
 #include "curl_printf.h"
 
@@ -144,3 +146,18 @@ bool Curl_auth_user_contains_domain(const char *user)
 
   return valid;
 }
+
+/*
+ * Curl_auth_ollowed_to_host() tells if authentication, cookies or other
+ * "sensitive data" can (still) be sent to this host.
+ */
+bool Curl_auth_allowed_to_host(struct Curl_easy *data)
+{
+  struct connectdata *conn = data->conn;
+  return (!data->state.this_is_a_follow ||
+          data->set.allow_auth_to_other_hosts ||
+          (data->state.first_host &&
+           strcasecompare(data->state.first_host, conn->host.name) &&
+           (data->state.first_remote_port == conn->remote_port) &&
+           (data->state.first_remote_protocol == conn->handler->protocol)));
+}

lib/vauth/vauth.h

@@ -54,6 +54,12 @@ struct gsasldata;
 #define GSS_ERROR(status) ((status) & 0x80000000)
 #endif
 
+/*
+ * Curl_auth_allowed_to_host() tells if authentication, cookies or other
+ * "sensitive data" can (still) be sent to this host.
+ */
+bool Curl_auth_allowed_to_host(struct Curl_easy *data);
+
 /* This is used to build a SPN string */
 #if !defined(USE_WINDOWS_SSPI)
 char *Curl_auth_build_spn(const char *service, const char *host,