MFC r272108: add missing file for building netmap as a module
MFH: sync the netmap code with the one in HEAD(enhanced VALE switch, netmap pipes, emulated netmap mode).See details in the log for svn 261909.
Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.Approved by: re (implicit)Sponsored by: The FreeBSD Foundation
add support to build netmap as a module,but for the time being keep it disconnected from the main build.MFC after: 3 days