Added support for FreeBSD accept filters

Added support for FreeBSD accept filters

ported from squid-2

Accept filters support will be enabled if the system defines the
SO_ACCEPTFILTER socket option.   A filter will be applied to Squid's
listen socket(s) if set with the 'accept_filter' directive in
squid.conf.

Typical use will be to apply the 'httpready' filter, which requires
loading the 'accf_http' kernel module.  The 'httpready' filter delays
passing new connection descriptors to Squid until a full HTTP request
is received.