Add Bridge MDB support

[troglobit]

• MDB
  • [x] Add static mdb entry (MAC and IP)
  • [x] Del static mdb entry (MAC and IP)
• Operational
  • [x] List mdb
  • [ ] List multicast router ports
  • [x] List multicast routers, per bridge or per VLAN
  • [x] List multicast status: on/off settings
  • [x] List per-port settings: flood, fast-leave, etc.
• Kernel
  • [ ] Finalize kernel patch(es) and upstream
• Documentation
  • [x] Update doc/networking.md
  • [x] Mention filtering limitation (snooping must be on)
  • [x] Describe mcast_flood and how it is good for MAC multicast, e.g. industrial fieldbus protocols

Note: due to how the MDB is implemented in the Linux bridge it is not possible to add MDB entries when multicast snooping is off. This needs to be documented clearly.

Tasks

• Verify adding ports to a bridge enables snooping on ports
• Verify toggling per-port multicast options take at runtime
• ...

[troglobit]

Remaining task, for operational router ports, moved to #395.