Add support for X-Forwarded-* HTTP headers alternates

When setting up Drupal for reverse proxies and crypto offloaders, it is now possible to configure all commonly used X-Forwarded-* headers, in addition to X-Forwarded-For which was already supported in before. In addition the Forwarded header specified in RFC 7239 is now supported.

Before

settings.php contained the following default values:

 /**
  * Set this value if your proxy server sends the client IP in a header
  * other than X-Forwarded-For.
  */
# $settings['reverse_proxy_header'] = 'X_CLUSTER_CLIENT_IP';

After

settings.php contained the following default values:

/**
 * Set this value if your proxy server sends the client IP in a header
 * other than X-Forwarded-For.
 */
# $settings['reverse_proxy_header'] = 'X_CLUSTER_CLIENT_IP';

/**
 * Set this value if your proxy server sends the client protocol in a header
 * other than X-Forwarded-Proto.
 */
# $settings['reverse_proxy_proto_header'] = 'X_FORWARDED_PROTO';

/**
 * Set this value if your proxy server sends the client protocol in a header
 * other than X-Forwarded-Host.
 */
# $settings['reverse_proxy_host_header'] = 'X_FORWARDED_HOST';

/**
 * Set this value if your proxy server sends the client protocol in a header
 * other than X-Forwarded-Port.
 */
# $settings['reverse_proxy_port_header'] = 'X_FORWARDED_PORT';

/**
 * Set this value if your proxy server sends the client protocol in a header
 * other than Forwarded.
 */
# $settings['reverse_proxy_forwarded_header'] = 'FORWARDED';