transproxy enabled on FreeBSD but not derivatives

Hi,

Noticed while investigating #18447, the transproxy feature is only 
enabled when __FreeBSD__ is defined, but only regular FreeBSD does that.
Please change this to __FreeBSD_kernel__ which is defined on derivatives
as well.

This enables the relevant options/validate__transproxy test on FreeBSD
derivatives.  (Tested on GNU/kFreeBSD).

Thanks.

Trac:
Username: stevenc99