C library: model usleep

[tautschnig]

We do not actually model "sleeping" in any way, just set the return values as specified in the man page.

• 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

Patch coverage has no change and project coverage change: -0.01% ⚠️

Comparison is base (d832f93) 78.90% compared to head (6a120a4) 78.90%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7901      +/-   ##
===========================================
- Coverage    78.90%   78.90%   -0.01%     
===========================================
  Files         1697     1697              
  Lines       195369   195369              
===========================================
- Hits        154151   154148       -3     
- Misses       41218    41221       +3     

see 17 files with indirect coverage changes

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