ifnet: Update and add comment; no functional changes

- Update various comment
- Remove no longer applied comment
- Add comment for ifaddr per-cpu container
- Add comment for per-cpu ifnet and ifaddr statistics
- Add comment for ifnet single serializer mode and multiple serializers
  mode
- Add pseudo code in comment for ifnet multiple transmit queue support
busdma: Update manpage about BUS_SPACE_UNRESTRICTED

Reported-by: ftigeot@
ip_output: Always panic if the rtentry is not owned by the current cpu

It has been on for several releases; make it mandatory now
ifq: Update comment; no functional changes
tso: Remove the pendantic tso_pullup functions

All of the drivers supporting TSO use the header lengthes stored in mbuf
pkthdr (csum_*hlen) to accelerate the transmit descriptors setup.
ethernet: Update comment; no functional changes
if: Update comment concerning subqueue; no functional changes