zdb: add extra -T flag to show histograms of BRT refcounts

[robn]

Motivation and Context

I was comparing my local BRT use against what fast dedup might get me[^1], and realised I would like a nice breakdown of refcount distribution. So I made one.

Description

Repurposes -TTT to show histograms. The full per-DVA list is now -TTTT.

How Has This Been Tested?

By hand:

$ ./zdb -TTT crayon
BRT: used 292M; saved 309M; ratio 2.05x
BRT: vdev 0: refcnt 12.2K; used 292M; saved 309M

BRT: vdev 0: DVAs with 2^n refcnts:
             1:  11788 ****************************************
             2:    645 ***
             3:     40 *
             4:      3 *
             5:      1 *

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] 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)
• [x] Documentation (a change to man pages or other documentation)

Checklist:

• [x] My code follows the OpenZFS code style requirements.
• [x] I have updated the documentation accordingly.
• [x] 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.
• [x] All commit messages are properly formatted and contain Signed-off-by.

[^1]: Turns out BRT is getting me basically everything I'd get from dedup. dedup would be marginally better at the cost of 11M unique entries. No thanks.

[robn]

Oh ugh, yeah. I meant to be counting DVAs. I'm not quite sure what I was thinking (at 11pm, probably not at all). Pushed what I think I meant.