[MultiSync] Optimizations and allow broadcast sync packets

- Update the Master to only send a sync packet every 16 frames.

- Update the Remote to adjust it's timing every time it receives a sync
  packet.  This changes the sync frequency to be driven by the master
  so remotes just keep in sync with whatever they receive.

- Allow Remote to startup in the middle of a sequence.  If the Remote
  receives a sync packet when it is not playing a sequence, it will open
  the sequence and seek to the correct position and start playback.
  This handles the situation where a Remote misses a start packet as
  well as allowing the Remote to be configured and restarted while the
  Master is playing without having to restart the Master's playback.

- Add the ability to broadcast to the local network.  The MultiSync
  config page has a new option to send to 'All Remotes'.  Checking this
  will use a broadcast destination of 255.255.255.255 to send to all
  remotes on the local network.   This allows users to check one box
  instead of having to check each individual Remote when there are a
  large number of Remotes.  It also reduces network traffic even
  further since we only transmit one sync packet for all Remotes instead
  of one sync packet for each Remote.
[Core] Send blanking data whenever fppd starts up.

A change in the multisync branch had blanking data only being sent
when we started up in player mode.  This commit changes fppd to
send blanking data no matter what mode fppd starts up in.