Redesign IPCP to use separate layer management and data transfer flows

[edugrasa]

Once shim-eth-vlan supports multiple flows between the same pair of applications, redesign the normal IPCP so that it uses separate flows for data transfer and for layer management. Requires #579

[vmaffione]

I think we should definitely allow the Shim IPCP over Ethernet to use a separate ethertype for management, so that the normal IPCP on top can allocate a management a flow and a data flow towards each neighbor on the LAN. Two flows, not more. This would really simplify a lots of things, first of all because it would remove the need for "the PCI header for management PDUs", and also the mgmt_write()and mgmt_read() system calls and related functionality. Also it would allow the normal IPCP to always try to allocate two flows on the N-1 DIF, independently on the fact that the N-1 DIF is a shim eth or not.