When implying keep-alive in client_connection(), remember that the client didn't

When implying keep-alive in client_connection(), remember that the client didn't

Fixes a regression introduced in jcc.c,v 1.265 that would
cause Privoxy to wait for additional client requests after
receiving a HTTP/1.1 request with "Connection: close" set
and connection sharing enabled.

With clients like curl which terminates the client connection
after detecting that the whole body has been received it doesn't
really matter, but with clients like FreeBSD's fetch the client
connection would be kept open until it timed out. Oops.

As it turns out the whole "upgrade server connection to
use keep-alive even when the client connection has to
be closed" currently doesn't work for other reasons,
so additionally add a XXX comment about it.