ifnet API: Change if_init() to take context argument

Some drivers, like iflib drivers, take a 'context' argument instead of a
ifnet argument, as a single interface may have multiple contexts.
Follow this scheme by passing the context argument down. Most drivers
will likely pass 'ifp' as the context.

Reviewed by: glebius
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D38102