Add provisions against VCL include loops

[dridi]

It should of course never happen, except when it unfortunately does, for example with unused files living in vcl_path nevertheless.

This breaks down the operations into 3 pipelines:

• find VCL files from vcl_path
• recursively find VCL includes
• output all VCL file names

Instead of one pipeline stream, this breakdown uses two temp files to keep track of loops.

Refs #96

[daghf]

Tested and reviewed - LGTM.