[PATCH] An svn_fspath__* API for '/'-prefixed paths

Any thoughts or concerns about this patch to add a path manipulation API
for '/fs/relative/style' aka '/repo/relative/style' paths ...

  svn_fspath__is_canonical()
  svn_fspath__join()
  svn_fspath__is_child()
  svn_fspath__basename()

so that we can stop abusing svn_uri_*() for this purpose, as discussed
in the thread <http://svn.haxx.se/dev/archive-2010-11/0277.shtml>?

If not, I'll commit it soon.

- Julian