Protect ifnet_list with pserialize facility
Use pserialize for other components
Implement refcount
Implement ifget, ifput and ifhold
Use ifget instead of ifunit
Pull copyout out of IFNET_LOCK and replace IFNET_LOCK with IFNET_RENTER
Replace IFNET_LOCK with IFNET_RENTER by using refcount
Describe ifnet locking rules and ifget/ifput/ifhold
Fix sysctl_dumpentry of rt
Introduce ifget_byindex
Tweak sysctl_dumpentry