Adds an over-approximation model for sysconf

[feliperodri]

• Each commit message has a non-empty body, explaining why the change was made.
• 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).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
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[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.47 🎉

Comparison is base (64bf1ca) 78.14% compared to head (830a60f) 78.61%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7780      +/-   ##
===========================================
+ Coverage    78.14%   78.61%   +0.47%     
===========================================
  Files         1697     1697              
  Lines       193629   193629              
===========================================
+ Hits        151309   152229     +920     
+ Misses       42320    41400     -920     

see 45 files with indirect coverage changes

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[tautschnig]

This is prone to arithmetic conversion failures for errno is signed.