xargs: add --show-limits diagnostics

[kevinburke]

Among other things, Debian maintainers use the output of xargs --show-limits to set --max-chars appropriately for subsequent commands.

Accept GNU-compatible --show-limits and print command-line size diagnostics before reading input or building a command. The output includes the Maximum length line parsed by Debian maintainer scripts.

The regression test links to Debian python3.13's
libPVER-minimal.prerm.in, where 3.13.5-2+deb13u2 parses xargs --show-limits, and to GNU findutils 4.10.0's xargs/xargs.c, where the diagnostic text is emitted.

Reuse the existing system command-size calculation for both the default limiter and the diagnostic output, and add an integration test for the parsed limit.

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.77%. Comparing base (c944cf7) to head (0ef2c9c).

Files with missing lines	Patch %	Lines
src/xargs/mod.rs	95.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #686      +/-   ##
==========================================
+ Coverage   91.76%   91.77%   +0.01%     
==========================================
  Files          31       31              
  Lines        6206     6240      +34     
  Branches      328      330       +2     
==========================================
+ Hits         5695     5727      +32     
- Misses        390      391       +1     
- Partials      121      122       +1     

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