Resolves a build error in Xcode:
> Build input file cannot be found: '.../CAuditToken.o'. Did you forget
to declare this file as an output of a script phase or custom build rule
which produces it?
- When a user performs an `su` the effective UID changes but the bootstrap
mach port does not, so that if container is running as `alice` from a
GUI login session, it's possible to `su bob` and continue running
container. While this doesn't pose a significant security risk as it's
necessary for Alice to know Bob's password and manually enter it with
`su`, this change closes the loophole by validating that client UID from
the caller's audit token matches that of the API server.