C library: fscanf for modern glibc

[tautschnig]

Recent versions of glibc rename fscanf (and also some others) to ones with a prefix of _isoc99, rendering our previous C library model useless. This commit adds a copy of the existing model with that new name, and also seeks to use it in a regression test.

The original goal, however, was to exercise a piece of code in string abstraction. Exercising this code now works, but results in an invariant failure. The test, therefore, remains marked KNOWNBUG.

• 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.
• n/a 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

Base: 77.82% // Head: 77.82% // No change to project coverage 👍

Coverage data is based on head (e0773e3) compared to base (2a08d59).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head e0773e3 differs from pull request most recent head f2ac528. Consider uploading reports for the commit f2ac528 to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7176   +/-   ##
========================================
  Coverage    77.82%   77.82%           
========================================
  Files         1578     1578           
  Lines       182242   182242           
========================================
  Hits        141824   141824           
  Misses       40418    40418           

Impacted Files	Coverage Δ
src/goto-programs/string_instrumentation.cpp	25.11% <100.00%> (ø)

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