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)