This moves the internal iam service from the posix backend so
that we can start implementing new iam services right in the auth
module.
The internal iam service has same behavior as before, but now
must be enabled with the --iam-dir cli option.
New single user service is the default when no other iam service
is selected. This just runs the gateway in single user mode with
just the root account.
This adds a context to the backend interface calls so that the backend
can enable request cancellation. This change isn't acutally implementing
any backend handling, but just putting the pieces into place to pass the
context to the backend.
Ben McClelland
jonaustin09