Log Message:
pkgtools/pkglint: update to 5.5.6

Changes since 5.5.5:

* Only offer explanations if an explainable diagnostic has actually
  been logged.

* Clean up code.

* Improve a few diagnostics.

* In any Makefile, treat documented variables as used. This prevents
  warning about defined but unused variables.

* Add support for some variables not previously known to pkglint.