C library: add creat, open, openat

[tautschnig]

These take pointer-typed arguments, which we should check for validity. Also add their large-filesystem version (with suffix 64).

• 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

All modified and coverable lines are covered by tests ✅

Project coverage is 77.96%. Comparing base (41496b9) to head (56669ea).

❗ Current head 56669ea differs from pull request most recent head d6f0ed8. Consider uploading reports for the commit d6f0ed8 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8271      +/-   ##
===========================================
- Coverage    77.96%   77.96%   -0.01%     
===========================================
  Files         1720     1720              
  Lines       188982   188982              
  Branches     18391    18391              
===========================================
- Hits        147347   147345       -2     
- Misses       41635    41637       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.