Rework if_gif(4) to use new encap_lookup_t method to speedup lookup
of needed interface when many gif interfaces are present.

Remove rmlock from gif_softc, use epoch(9) and CK_LIST instead.
Move more AF-related code into AF-related locations.
Use hash table to speedup lookup of needed softc. Interfaces
with GIF_IGNORE_SOURCE flag are stored in plain CK_LIST.
Sysctl net.link.gif.parallel_tunnels is removed. The removal was planed
16 years ago, and actually it could work only for outbound direction.
Each protocol, that can be handled by if_gif(4) interface is registered
by separate encap handler, this helps avoid invoking the handler
for unrelated protocols (GRE, PIM, etc.).

This change allows dramatically improve performance when many gif(4)
interfaces are used.

Sponsored by: Yandex LLC