Test multiple file support for the simple case in which all files
are in one directory.

Dependency graph:

     a.v    b.v     c.v
      \	    / \	    /
       \   /   \   /
        d.v     e.v
	  \    	/
	   \   /
            f.v

That is, d.v depends on a.v and b.v, and so on.

Some tests:

- first visit f.v and display the *Message* buffer in some other
  frame
- script f.v and watch the recompilation messages in the
  *Message* buffer
- Change now an arbitrary file (either from within emacs or
  simply do touch), retract the Require in f.v, and watch the
  compilation messages when you assert it again.

The following two problems have been fixed with the commit around 
2011-01-17 07:45:00 UTC.

The implementation in Proof General cvs at 2011-01-14 20:03:51 UTC 
has an embarrassing bug: Touching b.v causes recompilation of b.v
and d.v but not of e.v!

Another problem is the following: After a consistent compilation,
change b.v and recompile it outside of Proof General. Then script
f.v -- Proof General will not recompile d and e!