REMOTE_ADDR and option no_clobber_xff

In handler mode, set special header REMOTE_ADDR containing the IP address of the requesting client. Setting no_clobber_xff prevents X-FORWARDED-FOR from being touched in handler mode.

Proxy mode remains unchanged. REMOTE_ADDR is not set in that context, and X-FORWARDED-FOR is always set/replaced (no_clobber_xff has no effect on proxy mode).