Allow chunking the last HTTP response on a connection.

Allow chunking the last HTTP response on a connection.

Squid should avoid signaling the message end by connection closure
because it hurts message integrity and sometimes performance. Squid
now chunks if:

  1. the response has a body;
  2. the client claims HTTP/1.1 support; and
  3. Squid is not going to send a Content-Length header.

AFAICT, Squid used to exclude to-be-closed connections from chunking
because chunking support was added (trunk r10781) specifically to
optimize persistent connection reuse and closing connections were
incorrectly excluded as a non-interesting/out-of-scope case. And/or
perhaps we did not realize the dangers of signaling the message end
by connection closure.