Add --private-uids option to simplify default use

--private-uids option is introduced to simplify the use of virtual UID space.
A default host UID range is chosen to create the container rather than the user
having to specify a mapping.  If user specifies mappings using --uidmap, they
take precedence.  In either case, the semantics of -x remain the same:

    $ docker commit $(docker run -d -x --private-uids centos true) centos_private_uids
    $ docker run --private-uids -i -t centos_private_uids bash
    # cat /proc/self/uid_map
             0     100000      10000