flux-fsck: support --rootref option

[chu11]

Problem: Flux-fsck defaults to fsck-ing the checkpoint stored in the KVS backing store. There is no way to fsck check an alternate
root reference.

Support a new --rootref option where users can specify a specific root reference to begin with.

---

side notes:

• this is written independently of flux-content: support new checkpoints list command #6798, it can be considered a "prelude" PR where eventually users could specify the index of the checkpoint they actually want to fsck.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 83.83%. Comparing base (3323cf0) to head (3e71f0d).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/cmd/builtin/fsck.c	87.50%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6905      +/-   ##
==========================================
- Coverage   83.83%   83.83%   -0.01%     
==========================================
  Files         539      539              
  Lines       90283    90286       +3     
==========================================
- Hits        75693    75692       -1     
- Misses      14590    14594       +4     

Files with missing lines	Coverage Δ
src/cmd/builtin/fsck.c	77.94% <87.50%> (+2.94%)	⬆️

... and 8 files with indirect coverage changes

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