changes to net collection's URL code

[Shriram Krishnamurthi <shriram@cs.rice.edu>]

The net collection's URL code has been cleaned up in a few ways,
eliminating one really, really obscure bug (really).  The most
important benefit to users from these changes is that you can now
specify filesystems in your file: URLs.  If your URL is of the form

  file:/foo/bar/baz

and "foo" is one of the entries returned by `filesystem-root-list'
(foo is more likely to be something like "C:"), then this is assumed
to be a reference to the foo root; otherwise, foo/bar/baz is assumed
to be relative to the first entry in the values returned by
`filesystem-root-list'.

See the doc.txt for more specific documentation.  Note that the
unixpath->path name is now deprecated.

'shriram