[#1234][Forge] Be more selective when sending reset packets.

Partial fix for #1234. The reset packet is not sent unless needed. All vanilla server networks will be able to run without worrying about Forge versions (however, please note that these server owners/plugins will not be able to get the users' mod list out either.)

If the server owner runs a Forge network with a vanilla lobby, then Forge users with builds less than 1209 will still crash with the FML¦HS error message.

This commit redefines when to send the reset packet, so it always send the packet when going from modded -> anything, sends the packet later when going from vanilla -> modded, never sends it on a vanilla -> vanilla switch.