Add pretty print command

Thanks to Tommy Andersen <andersentommy@gmail.com> for the idea!

See https://github.com/Barium/killer-sudoku-solver.git

Only collect filenames with 4 digits when testing

Be a little bit more verbose when tests fail

Do not over-allocate

Looks like a left-over from a malloc call.

Add man pages

Update README

* Be more precise when describing the input format
* Fix a typo
* Add link to blog post

Run the linter at the end of the CI build

Make scan-build executable configurable

Consume all input after detecting an syntax error

Increment pointer before NUL check

No functional change. Saves one iteration and expresses the intend more
clearly.

Check for the correct error message in tests

The program could fail for numerous reasons. Check the error message to
ensure it really is the error we expected.

Use more intuitive logic for candidate list

Using 'false' to set a candidate feels wrong and has probably historical
reasons.

Use cells instead of indirection via coordinates

Rewrite next hint function

Rename variable

Make it more obvious it is a coordinate for a cell.

Rename function

Fix a typo in build job's task names

Document minimal C and POSIX version required

Revert "Suppress cppcheck warnings"

This reverts commit 449806f36689446ae6853b1ff9da47cc522bc9a0.

Suppress cppcheck warnings