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