More constness/C23 fixes [master]

[hhoffstaette]

Description

This is #169 but for the master branch.

Type of Change

• Bug fix (targets 0.4.x)
• New feature (targets master)
• Documentation update
• Other (please describe)

Testing

make check:

============================================================================
Testsuite summary for patchutils 0.4.5
============================================================================
# TOTAL: 217
# PASS: 209
# SKIP: 0
# XFAIL: 8
# FAIL: 0
# XPASS: 0
# ERROR: 0
============================================================================

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.32%. Comparing base (0e9ce17) to head (b24a87b).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #168 +/- ##
=======================================
Coverage 86.32% 86.32% =======================================
Files 15 15 Lines 8118 8118 Branches 1662 1662 =======================================
Hits 7008 7008 Misses 1110 1110 

Flag	Coverage Δ
unittests	86.32% <100.00%> (ø)

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