HTTP/1.1 compliance: Stop using Proxy-Connection header

HTTP/1.1 compliance: Stop using Proxy-Connection header

The Proxy-Connection header is not part of any HTTP standard. It was added
by Netscape to differentiate persistent connections to intermediary proxies
but that duty has been formally superceded by the Connection: header.

This compliance update makes Squid stop sending Proxy-Connection on outbound
requests. Starts consistently using Connection: header instead.

The Proxy-Connection header is also ignored on HTTP-strict builds.
For compatibility we must do a small violation and drop it as a hop-by-hop
header despite strct-mode technically being required to pass it through.

For origin server connections the non-strict builds will retain the
status-quo: interpret it, but treat it as an HTTP/0.9 thing to be
upgraded to HTTP/1.1 Connection:.

squidclient is also fixed not to send it.