Don't always use default mapping to "nobody".

Don't always use default mapping to "nobody".

Signed-off-by: Kevin Coffman <kwc@citi.umich.edu>
We've been ignoring all mapping errors and instead mapping to a "nobody" user
or group.

This is arguably OK for the cases where we're returning a value to the user
(so, id->name mapping on the server or name->id mapping on the client).

But it's a disaster in the other direction (id->name on the server or id->name
on the client): for example, a chown to an unknown user should *not*
automatically be translated into a succesful chown to "nobody".

This patch fixes that problem on the server side.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>