Add -x flag to optimize away UID translation

If -x flag is not set, UIDs of the files in the image are assumed to
match the specified UID mappings and no UID translation is performed.
Images with UIDs already translated can be produced by simply committing
a container created with -x flag:

	$ docker commit $(docker run -d -x centos true) centos-x
	$ docker run -i -t centos-x bash