MFC r339639

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:    gnn (mentor)
Differential Revision:  https://reviews.freebsd.org/D17364