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.
MFC r273090:
Move memset under ifdef INET6.
MFC r273091:
Add more ifdefs. SIOC*_IN6 are defined only with INET6.
MFC r273121:
Add inet/inet6 to the dependency list. Without them if_gif is useless.
MFC r273209 by bz:
After r273087,r273090,r273091,r273121 changes to gif(4) try to fix
NOIP builds for real.
MFC r273587:
Remove redundant check and m_pullup() call.
7a82e24551