FreeBSD: Organize sysctls

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Motivation and Context

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

vfs.zfs.arc_free_target
vfs.zfs.arc_no_grow_shift

instead of:

vfs.zfs.arc.free_target
vfs.zfs.arc.no_grow_shift

In fixing this I noticed that most sysctl handlers for ZFS module parameters were not marked mpsafe. All the parameter handlers implement their own locking if needed and do not require the global Giant lock.

I also did some general cleanup including organization of sections, not passing unneeded arguments to handlers, using the correct types for local temporaries, etc.

Description

• Mark ZFS_MODULE_PARAM_CALL handlers as MPSAFE.
• 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.
• 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.

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

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Xref: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=261212

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I've also changed several legacy sysctls that were CTLFLAG_RW to now match the corresponding new sysctls as CTLFLAG_RWTUN.