Document and test thread-specific unwindset option

[tautschnig]

We had working support for thread-specific limits as of 7f7e3c4,
but seemingly never exposed this in documentation.

Also cleaned up the command-line parsing code to avoid repeated
find/rfind operations.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6510 (a9ae449) into develop (23d90c8) will increase coverage by 0.00%.
The diff coverage is 56.83%.

@@           Coverage Diff            @@
##           develop    #6510   +/-   ##
========================================
  Coverage    76.10%   76.10%           
========================================
  Files         1548     1548           
  Lines       166325   166328    +3     
========================================
+ Hits        126576   126584    +8     
+ Misses       39749    39744    -5     

Impacted Files	Coverage Δ
jbmc/src/jdiff/jdiff_parse_options.h	100.00% <ø> (ø)
src/analyses/goto_check.cpp	62.50% <ø> (ø)
src/ansi-c/ansi_c_internal_additions.cpp	90.36% <ø> (ø)
src/crangler/scanner.l	63.63% <ø> (ø)
src/goto-instrument/function_assigns.cpp	0.00% <0.00%> (ø)
src/goto-instrument/function_assigns.h	0.00% <0.00%> (ø)
.../goto-instrument/goto_instrument_parse_options.cpp	69.14% <ø> (ø)
...rc/goto-instrument/goto_instrument_parse_options.h	100.00% <ø> (ø)
src/goto-instrument/havoc_loops.cpp	0.00% <0.00%> (ø)
src/goto-instrument/unwindset.h	100.00% <ø> (ø)
... and 19 more

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