follows BSD/OS practice and ucd-snmp code (FreeBSD does it for specific
interfaces only).
was: if_lastchange get updated on every packet transmission/receipt.
now: if_lastchange get updated when IFF_UP is changed.
the link level name for the interface (ifp->if_sadl) is allocated
before ifp->if_addrlen is initialized, which could lead to allocating
too little space for the link level address.
Do this by splitting allocation of the link level name out of
if_attach() and into if_alloc_sadl(), which is normally called
by functions like ether_ifattach(). Network interfaces which
don't have a link-specific attach routine must call if_alloc_sadl()
themselves (example: gif).
Link level names are freed by if_free_sadl(), which can be called
from e.g. ether_ifdetach(). Drivers never need call if_free_sadl()
themselves as if_detach() will do it if it is not already done.
While here, add the ability to pass an AF_LINK address to
SIOCSIFADDR in ether_ioctl() (this is what caused me to notice
the problem that the above fixes).
allow the user to set and use the internal baud rate generator
fix the transmission ring logic to support more than 1 frame per interrupt
add autodetection of the base clock frequency.
cleanup the receive ring logic
support dynamically resizing the low-water mark on the fifo in response
to buffer underruns on transmit.
