Move initialization of virtif's DPDK underpinnings to module init.

To do this, we create a new VIFHYPER hook, and have the module
handling code call it.  globalinit is then renamed to VIFHYPER_INIT.
Bump the VIFHYPER interface revision/date.
Check for VIFHYPER interface compatibility.
Clean up unused code.
Take this out to the trash as well.
Merge in rump-pktgenif's virtif interface/implementation as of 8285e0df.
Update to support the newest virtif hypervisor interface:

- Don't currently report any capabilities.
- Accept checksum related metadata, but don't do anything with it.
- Respect the new expectations of VIF_MBUF_EXTALLOC (from pooka@).
More ignores.
Make the DPDK port dynamic, based on the interface name.