Merge r278874, r278925, r278868

• Improve INET/INET6 scope.
• style(9) declarations.
• Make couple of local functions static.
• Even more fixes to !INET and !INET6 kernels. In collaboration with pluknet
• Toss declarations to fix regular build and NO_INET6 build.

Differential Revision: https://reviews.freebsd.org/D2823
Reviewed by: gnn