Unicode normalization needs to be applied to filenames in more cases

[davidsarah]

Currently, the CLI normalizes filenames to NFC when listing the contents of a local directory in listdir_unicode (used by tahoe cp and tahoe backup, but that is the only point at which filenames are normalized.

So, unnormalized filenames can get into Tahoe directories via CLI arguments, SFTP, FTP, and the WUI.

This is a forward-compatibility issue because, if we have any non-NFC filenames stored in Tahoe directories that we need to maintain compatibility with, then we would have to normalize when reading filenames out of Tahoe directories and not just when putting filenames into them.