There's an ongoing project to bring some kind of path selection

There's an ongoing project to bring some kind of path selection
mechanism to the ENGINE framework.  This means there there are going
to be new functionality for the DSO part, and ultimately some way of
merging two file specifications together.

This commit places the merging code into the repository.  It's
currently not used anywhere, and hasn't been tested at all.  It may be
full of errors, including syntactical ones.  Those will be fixed as
promptly as possible.