Add a URI parameter to FTPFileSystem and FTPPath

Needed to correctly build Path's .toURI().
FTPPath: implement .to{URI,AbsolutePath}()

For URI creation, we take the normalized form of the path and remove the
trailing dots before we append it to the base URI. We don't want to be able to
"escape" the root filesystem defined for us by the URI.

For the absolute path, we only prepend a / to the path if there isn't one. It
_should_ work...
FTPPath: implement .equals()/.hashCode() plus .toString()