FreeBSD: Organize sysctls

FreeBSD: Organize sysctls

FreeBSD had a few platform-specific ARC tunables in the wrong place:

- Move FreeBSD-specifc ARC tunables into the same vfs.zfs.arc node as
  the rest of the ARC tunables.
- Move the handlers from arc_os.c to sysctl_os.c and add compat sysctls
  for the legacy names.

While here, some additional clean up:

- Most handlers are specific to a particular variable and don't need a
  pointer passed through the args.
- Group blocks of related variables, handlers, and sysctl declarations
  into logical sections.
- Match variable types for temporaries in handlers with the type of the
  global variable.
- Remove leftover comments.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #13756