Support for user namespaces

This exposes UID namespace support which is recently included in the kernel
via a new command line option (--uidmap) to run an application within
a virtual UID namespace where the application could be a root in the
container but unprivileged on the host.

Docker-DCO-1.1-Signed-off-by: Dinesh Subhraveti <dineshs@altiscale.com> (github: dineshs-altiscale)