Add support for accf_http & accf_data on FreeBSD

This patch adds support for accf_http & accf_data on FreeBSD platforms that support it (it's been available since 2000, so almost all should).

accf_data buffers connections until data arrives, accf_http goes a step further and buffers connections until a complete HEAD or GET request has arrived. This should increase performance on busy machines.
See: 
http://www.freebsd.org/cgi/man.cgi?query=accf_data&sektion=9&apropos=0&manpath=FreeBSD+9.1-RELEASE
http://www.freebsd.org/cgi/man.cgi?query=accf_http&apropos=0&sektion=0&manpath=FreeBSD+9.1-RELEASE&arch=default&format=html

I've also updated the Makefiles, docs, etc. I've set the option to `enabled' by default if accept filters are available, this is also what Apache does.