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.

Move memset under ifdef INET6.

Add more ifdefs. SIOC*_IN6 are defined only with INET6.

Add inet/inet6 to the dependency list. Without them if_gif is useless.

After r273087,r273090,r273091,r273121 changes to gif(4) try to fix
NOIP builds for real.

Remove redundant check and m_pullup() call.