WebSession: reintroduce configurable IP checking

• Commit 314db6ae6ddaa6d14c7080cc40cbf5a882e4f224 removed support for CFG_WEBSESSION_IPADDR_CHECK_SKIP_BITS config variable, but this might still be useful in some particular proxy condition (e.g. if a user is behind a multi-node round-robin proxy). Moreover, now that authenticated sessions are always served via HTTPS (if possible), IPADDR check can be disabled.