core.packet: require explicit initialization of packets freelist

This is to ensure that supervisors loading the core.packet module will not
populate their SHM roots.

src/core/main.lua

@@ -158,7 +158,7 @@ function initialize ()
    _G.engine = require("core.app")
    _G.memory = require("core.memory")
    _G.link   = require("core.link")
-   _G.packet = require("core.packet")
+   _G.packet = require("core.packet"); _G.packet.initialize()
    _G.timer  = require("core.timer")
    _G.main   = getfenv()
 end

src/core/packet.lua

@@ -104,8 +104,13 @@ end
 
 local packet_allocation_step = 1000
 local packets_allocated = 0
-local packets_fl = freelist_create("engine/packets.freelist")
-local group_fl -- Initialized on demand.
+ -- Initialized on demand.
+local packets_fl, group_fl
+
+-- Call to ensure packet freelist is enabled.
+function initialize ()
+   packets_fl = freelist_create("engine/packets.freelist")
+end
 
 -- Call to ensure group freelist is enabled.
 function enable_group_freelist ()