Allow upgrading from HTTP/1.1 to other protocols (#481)

Allow upgrading from HTTP/1.1 to other protocols (#481)

Support admin-authorized HTTP Upgrade-driven (RFC 7230 Section 6.7)
protocol switching. The new http_upgrade_request_protocols configuration
directive allows admin to control what client Upgrade offer(s) are
forwarded to the server. Squid does not automatically check whether the
server selection matches one of the client offers, but the admin can
deny unacceptable server responses using http_reply_access.

By default, Squid still does not forward any Upgrade headers,
effectively blocking an upgrade attempt.

Squid itself does not understand the protocols being upgraded to and
participates in the upgraded communication only as a dumb TCP proxy.

This is a Measurement Factory project.