stop using REMOTE_ADDR

stop using REMOTE_ADDR

Everywhere that REMOTE_ADDR was used, a session object is available, so
instead use its remote_addr method.

In IkiWiki::Receive, stop setting a dummy REMOTE_ADDR.

Note that it's possible for a session cookie to be obtained using one IP
address, and then used from another IP. In this case, the first IP will now
be used. I think that should be ok.