new BORG_UNITS env var: si / iec / raw size formatting, #5513

[ThomasWaldmann]

Fixes #5513.

BORG_UNITS determines how borg formats sizes in human-readable output:

• si (default): decimal units, e.g. 1.23 MB (1kB = 1000B)
• iec: binary units, e.g. 1.18 MiB (1KiB = 1024B)
• raw: exact byte counts, e.g. 1234567 B

raw is what #5513 asked for: sizes that a script (e.g. for monitoring) can parse without
having to deal with scaled values and different units. It applies to all human-readable size
output, e.g.:

$ BORG_UNITS=raw borg info arch1 | tail -1
Original size: 993726 B

$ BORG_UNITS=raw borg -v compact --stats
Source data size was 993691 B in 68 files.
Deduplicated size is 993691 B.
Repository size is 524037 B in 69 objects.

Note that borg compact --stats uses precision=0, so it printed e.g. Repository size is 2 MB before - with BORG_UNITS=raw the exact value is not lost.

BORG_IEC was removed, use BORG_UNITS=iec instead. An invalid BORG_UNITS value is
warned about (once) and ignored.

Implementation notes:

• get_size_units() resolves the units and format_file_size() uses them, so the whole iec
  argument plumbing through Archive / Statistics / ArchiveFormatter / Cache /
  ArchiveChecker / ArchiveGarbageCollector and use_iec_units() could be removed. Beside
  being less code, this also makes the setting effective at the call sites that never passed
  iec=use_iec_units(), e.g. borg repo-space.
• raw rounds floats (some callers format throughput values).
• The man pages are not updated here: scripts/make.py build_man currently regenerates all 48
  of them (they are stale for unrelated reasons, e.g. the return codes section and
  borg-webdav), which would just add noise to this PR.

Not part of this PR, but noticed while looking at the issue: for the repository size, which is
what the issue author actually wanted to monitor, there is still no machine-readable output at
all - borg compact --stats and borg analyze only log human-readable text (no --json), and
borg repo-info does not report sizes. Archive sizes are fine (borg info --json,
borg list/repo-list --format "{size}" give byte counts).

[codecov]

Codecov Report

❌ Patch coverage is 98.14815% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 85.84%. Comparing base (28de45c) to head (3a864cd).
⚠️ Report is 7 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/archive.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9977      +/-   ##
==========================================
+ Coverage   85.83%   85.84%   +0.01%     
==========================================
  Files          95       95              
  Lines       17034    17056      +22     
  Branches     2607     2614       +7     
==========================================
+ Hits        14621    14642      +21     
- Misses       1673     1674       +1     
  Partials      740      740              

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