netmap: Add an option to specify the NUMA affinity of a registered port

[markj]

Summary

Untested so far, just interested in feedback before I add a
corresponding wrapper for libnetmap.

[vmaffione]

Looks good, but maybe I would use more evocative names such as:

`
netmap_get_local_allocator()
netmap_get_na_local_allocator()
`

in contrast with the "global" allocator.
It's not necessary that the names start with netmap_mem, since there are more functions that do not start with that.