sys/net/if_bridge: support non-INET kernels

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1159

(cherry picked from commit 65767e6126a7e92bc76561b4ffd005cf85ba525e)

if_bridge: Minor style fixes

And more comments on the #ifdef INET blocks to improve readability.

While here, revert the order of two prototypes to produce minimal diff
compared to stable branches.

(cherry picked from commit 73585176ffd84c13d68cad67c2ca81643f09075c)