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.  Parsing errors and warnings *do* show the included filename.

Absolute and relative filenames work; acceptance test (uses relative filenames) segfaults, but it works in standalone testing (see sample policy in Redmine).  I can't find the problem in my testing, and believe it's not due to my code.