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)
(cherry picked from commit d45f1f5193420fac7f9a64c0455991a80e7cb89a)