netmap: align codebase to the current upstream (760279cfb2730a585)

Changelist:

• Replace netmap passthrough host support with a more general mechanism to call TXSYNC/RXSYNC from an in-kernel event-loop. No kernel threads are used to use this feature: the application is required to spawn a thread (or a process) and issue a SYNC_KLOOP_START (NIOCCTRL) command in the thread body. The kernel loop is executed by the ioctl implementation, which returns to userspace only when a different thread calls SYNC_KLOOP_STOP or the netmap file descriptor is closed.
• Update the if_ptnet driver to cope with the new data structures, and prune all the obsolete ptnetmap code.
• Add support for "null" netmap ports, useful to allocate netmap_if, netmap_ring and netmap buffers to be used by specialized applications (e.g. hypervisors). TXSYNC/RXSYNC on these ports have no effect.
• Various fixes and code refactoring.

Sponsored by: Sunny Valley Networks
Differential Revision: https://reviews.freebsd.org/D18015