Add nvram user.

This is intended to be used by user space daemons that are part of
access-controlled NVRAM HAL implementations for Brillo. Because there
might be sensitive data flowing through the daemon, we'd like to run
it as a separate user.

BUG: 27764637
Change-Id: If6ad7923af3c650963ee9e926f732001b6a8cb5d