netmap: align codebase to upstream version v11.4

netmap: align codebase to upstream version v11.4

Changelist:
  - remove unused nkr_slot_flags
  - new nm_intr adapter callback to enable/disable interrupts
  - remove unused sysctls and document the other sysctls
  - new infrastructure to support NS_MOREFRAG for NIC ports
  - support for external memory allocator (for now linux-only),
    including linux-specific changes in common headers
  - optimizations within netmap pipes datapath
  - improvements on VALE control API
  - new nm_parse() helper function in netmap_user.h
  - various bug fixes and code clean up

Approved by:	hrs (mentor)