Redmine#3323: @include proposal

Proposal to implement https://cfengine.com/dev/issues/3323

Briefly, you say `@include "filename"` and it gets included verbatim at the lexing stage, before the parser ever sees it.  This is noted in the `-d` output through `ParserDebug` but not in regular CFEngine logging.  So logging needs to be improved.

Absolute filenames work.  Relative filenames should work; acceptance test (uses relative filenames) segfaults, but it works in standalone testing (see sample policy in Redmine).  Not sure why, probably something to do with the path manipulations.  Also needs to be fixed.