Remove debug.zfs_flags in favor of the new vfs.zfs.debug_flags.
Replace TUNABLE_INT with CTLFLAG_RWTUN.

Submitted by:	avg (debug.zfs_flags removal), smh (TUNABLE_INT replacement)
- Provide if_get_counter() method for vtnet(4).
- Do not accumulate statistics on every tick.
- Accumulate statistics in vtnet_setup_stat_sysctl()
  and in vtnet_get_counter().

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