Some more basic code in FtpFileSystemProvider

We don't support byte channels. As the javadoc dictates, we therefore override
.newInputStream() and .newOutputStream().

Make the latter throw an IllegalStateException. Given that we declare the
filesystem to be readonly, we should not hit that method.
Implement FtpFileStore and the rest of FtpFileSystem

Now, there remains the "slight" problem of FtpFileSystemProvider...