This patch implements configuration option "ip-transparent" on FreeBSD (IPv4 and IPv6). Please notice the following:

1. There's some refactoring involved. Different platforms use different macro names for the same socket option, and the previous code block simply repeated multiple times the same setsockopt() lines for each different name. This wasn't scalable; this patch uses one linear implementation based on generic macro name, which are resolved to platform-specific socket option names by some pre-processor code upfront. (see comment in code for more details). To add support for a platform with yet a different socket option name, simply define macros the respective macros accordingly.
2. binding both IPv4 and IPv6 addresses is supported on FreeBSD. Linux doesn't seem to support this option on IPv6 programmatically yet, so that's not added. When it is, the pre-processor code makes it easy to add such support: simply add definitions for NSD_SOCKET_OPTION_TRANSPARENT6 and NSD_SOCKET_OPTION_TRANSPARENT_OPTLEVEL6 accordingly.
3. nsd.conf describes the ip-freebind and ip-transparent options, seemingly for the same purpose. They are also implemented with same code. This patch only refactors the ip-transparent implementation. Admittedly we did not put much effort into reversing what difference :) if equivalent, consider removing one of the options; if not, consider clarifying the difference in the man page.