Fix NETMAP_API, NETMAP_MIN_API (netmap.rs)

[hirrolot]

The current API versions of NETMAP are defined as the following:

#define	NETMAP_API	14		/* current API version */

#define	NETMAP_MIN_API	14		/* min and max versions accepted */
#define	NETMAP_MAX_API	15

[polachok]

We have to make sure all struct definitions are compatible with this version before bumping. I think bindgen would be the best way forward.

[hirrolot]

Why don't we just copy-paste the NETMAP's headers into this crate and invoke bindgen?