Minor overhaul and add locking.
Minor overhaul and add locking.

o replace magic constants with #defines (e.g. ETHER_ADDR_LEN)
o move mib variables to net.link.ether.bridge with backwards compatible
  entries for well-known items maintained under BURN_BRIDGES
o revamp debugging support so it is conditioanlly compiled with BRIDGE_DEBUG
  (on currently) and runtime controlled by net.link.ether.bridge.debug
o change timeout to MPSAFE callout
o optimize lookup for common case of two interfaces
o optimize forwarding path to take IFNET lock only when needed
o make boot-time printf dependent on bootverbose
o sundry style changes (ANSI decls, extraneous spaces, etc.)

Sponsored by:	FreeBSD Foundation

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