vfs: use None for unknown uid/gid

vfs: use None for unknown uid/gid

This means we show '?' instead of 0 for unknown UIDs when
numeric output is requested, as it was before.

This also uncovered a forgotten bytes annotation for the
"unknown" string ('?' should be b'?').

Somehow, this new behaviour (of printing 0 instead of ?)
also got quite enshrined in the test suite, fix that too.

And finally, on python 2, fuse doesn't accept None in the
stat struct (but does on python 3, go figure).

Fixes: f76c37383ddb ("Remove vfs (replaced by vfs2)")
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>