Update fuse to translate uids and gids to/from the user namspace
of the process servicing requests on /dev/fuse. Any ids which do
not map into the namespace will result in errors. inodes will
also be marked bad when unmappable ids are received from
userspace.

Due to security concerns the namespace used should be fixed,
otherwise a user might be able to gain elevated privileges or
influence processes that the user would otherwise be unable to
manipulate. Thus the namespace of the mounting process is used
for all translations, and this namespace is required to be the
same as the one in use when /dev/fuse was opened.

Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Serge H. Hallyn <serge.hallyn@ubuntu.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>