MFC r339639:

netmap: align codebase to the current upstream (sha 8374e1a7e6941)

Changelist:

• Move large parts of VALE code to a new file and header netmap_bdg.[ch]. This is useful to reuse the code within upcoming projects.
• Improvements and bug fixes to pipes and monitors.
• Introduce nm_os_onattach(), nm_os_onenter() and nm_os_onexit() to handle differences between FreeBSD and Linux.
• Introduce some new helper functions to handle more host rings and fake rings (netmap_all_rings(), netmap_real_rings(), ...)
• Added new sysctl to enable/disable hw checksum in emulated netmap mode.
• nm_inject: add support for NS_MOREFRAG

Approved by: re (gjb)