Update FreeBSD tunables

[ghost]

Motivation and Context

Description

Remove some obsolete legacy compat, rename some misnamed, and add some
missing tunables for FreeBSD.

How Has This Been Tested?

ZTS on FreeBSD

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)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the ZFS on Linux 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.

[codecov-io]

Codecov Report

Merging #10203 into master will increase coverage by 0.35%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10203      +/-   ##
==========================================
+ Coverage   79.49%   79.84%   +0.35%     
==========================================
  Files         388      290      -98     
  Lines      123392    83452   -39940     
==========================================
- Hits        98088    66635   -31453     
+ Misses      25304    16817    -8487     

Flag	Coverage Δ
#kernel	79.84% <ø> (-0.16%)	⬇️
#user	?

Impacted Files	Coverage Δ
module/zfs/ddt.c	83.16% <ø> (-5.09%)	⬇️
module/zfs/txg.c	91.34% <ø> (-2.38%)	⬇️
module/icp/asm-x86_64/aes/aeskey.c	0.00% <0.00%> (-100.00%)	⬇️
module/icp/algs/modes/gcm_generic.c	11.11% <0.00%> (-88.89%)	⬇️
module/zfs/vdev_raidz_math_sse2.c	14.39% <0.00%> (-84.85%)	⬇️
module/icp/algs/aes/aes_impl_generic.c	0.82% <0.00%> (-69.55%)	⬇️
module/icp/algs/skein/skein_block.c	31.94% <0.00%> (-68.06%)	⬇️
module/icp/algs/aes/aes_impl_x86-64.c	40.00% <0.00%> (-60.00%)	⬇️
module/icp/algs/edonr/edonr.c	45.72% <0.00%> (-42.59%)	⬇️
module/zfs/gzip.c	46.15% <0.00%> (-38.47%)	⬇️
... and 278 more

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