Patches for FreeBSD

Two things:
* make it possible to change PREFIX and add a --prefix option for _GenerateDissectors.pl
* a /proc filesystem does not exist for FreeBSD (that is, there's a procfs which is optional, but it does not has /proc/net/tcp)

Also I added a --display switch for the interceptor.pl, so people can use different displays with --display 127.0.0.1:8