Added packet.tenure() to inhibit reuse by deref().
Refactored 'link' code.

link_ring.{h,lua} is the low-level packet ring data structure.
app.lua contains the higher level bits.
packet.allocate(): error when no packets available.
packet.lua: Trivial numeric constant update.
app.lua: Added transfer() as convenient transmit-and-deref.
(Apps are now responsible for performing deref.)
app.lua: Messy experimental optimization changes.