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