protocols: init with standard SYSINIT(9) or VNET_SYSINIT

The historical BSD network stack loop that rolls over domains and
over protocols has no advantages over more modern SYSINIT(9).
While doing the sweep, split global and per-VNET initializers.

Getting rid of pr_init allows to achieve several things:
o Get rid of ifdef's that protect against double foo_init() when

both INET and INET6 are compiled in.

o Isolate initializers statically to the module they init.
o Makes code easier to understand and maintain.

Reviewed by: melifaro
Differential revision: https://reviews.freebsd.org/D33537