Overhaul if_gif(4):
o convert to if_transmit;
o use rmlock to protect access to gif_softc;
o use sx lock to protect from concurrent ioctls;
o remove a lot of unneeded and duplicated code;
o remove cached route support (it won't work with concurrent io);
o style fixes.

Reviewed by: melifaro
Obtained from: Yandex LLC
MFC after: 1 month
Sponsored by: Yandex LLC