[plt-scheme] file: urls & their relation to path names

It looks like there is a little bit of under-specification (or maybe a
mistake?) in rfc 3986 about dealing with file: urls. For example,
should the path:

  /home/robby/tmp.ss

be represented in the file: url as:

  file:///%3f/home/robby/tmp.ss

or as:

  file:///home/robby/tmp.ss

?

That is, should the first path element (namely the path that
corresponds to the root of the filesystem, aka (build-path "/"))
quoted as a special path in the file: url, or not. 

If not, what does that / correspond to on filesystems where the root is
not "/", like windows? Should the file: url that points to the file
"tmp.txt" in the c: drive be:

  file:///c:/tmp.txt

That is, should it have a forward slash ala the rfc to indicate
"separate path segments here" or should it have the backwards slash
like this:

  file:///c:\tmp.txt

which would, as far as the url is concerned, be treated as a file: url
with only a single path element "c:\\tmp.txt".

I think I'm going to go with the somewhat hackish behavior of
specialcasing the first path segment and, if it is "/" I'll just drop
it, but otherwise split of the path into the its segments from
mzscheme's point of view and then put them together wish "/"s in
between.

  file:///home/robby/tmp.ss
  file://c:/tmp.txt

(This change will also affect how call/input-url works for file: urls.)

Robby