Some validation functions only validated, while some would actually make
a tweaked version and return it. This caused some issues where the
function was called incorrectly (it returns the new value, which
evaluates to True, but if the value is different this should be
interpreted as a failed validation).

The value-returning functions are now prefixed by `fix_` and have a
purely-validating counterpart.

In addition, don't bother sanitizing document filenames for filesystem
use, just make sure they aren't very long. Sanitize them when we
actually write to disk (e.g. in convert.py).