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
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