Revise netisr2 interface some more:

• Add public APIs to query available workstream counts and CPU IDs for them (netisr2_get_cpucount(), netisr2_get_cpuid()).

• Provide default flow2cpu inmplementation as netisr2_default_flow2cpu, and install it if the protocol doesn't register one.

• Add uintptr_t source argument to netisr2_queue() and netisr2_dispatch(), allowing to caller to provide additional information about a source that might be used to improve parallelism by constraining ordering. Should be set to 0 if none is identified (i.e., maximum ordering).

• If the protocol doesn't provide a flow ID conversion function and there isn't one on the packet, use the source to order.