rc files searched for upwards from current directory

[Peter TB Brett]

 affects geda
 tag libgeda
 importance wishlist
 done

It's fairly common to have a project directory structure like this:

  ./ # top-level schematics
  ./subcircuits/ # hierarchical sub-sheets
  ./symbols/ # project-specific symbol files

At the moment, it's necessary to have 'gafrc' and 'gschemrc' files in
all directories from which gschem might load schematics and/or symbols,
in order to make sure that all the libraries and behavioural options etc
are set up properly.

If gEDA can't find any relevant config files in the working directory,
it should recurse upwards from the current directory until it finds a
relevant rc file. It should probably stop at filesystem boundaries.
This change would make things a lot simpler for multi-directory
projects, as it would save duplication in rc files.