Add splint, modified some files to pass checks

[mdeweerd]

This adds splint as a workflow.

• do_splint.sh (with splint installed) with arguments will check only those files without disabling warnings.
  Without arguments it checks "all" files (a lot of files were excluded).
• Except for a duplicate function warning, I disabled all messages and files that could not be parsed.
• Files are excluded by grepping the list of 'c' files resulting from a find (in do_splint.sh). So to enable parsing for a file in the workflow, that has to be updated.

Most of the time parsing fails because variables are not defined at the start of a block.
I adjusted some files for that and in 'include/abuf.h' I added a '#ifndef S_SPLINT_S' to work around the elipsis limitation of splint.