tarsum: TarSum is not the interface

don't export the exsisting TarSum struct and call the interface 'TarSum'
instead.

Signed-off-by: Vincent Batts <vbatts@redhat.com>
Ensure container names start with a-zA-Z0-9

Closes #8012.

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)
Merge pull request #8014 from jfrazelle/8012-prevent-container-names-starting-dash

Ensure container names start with a-zA-Z0-9
Merge pull request #7983 from vbatts/vbatts-tarsum_switch_names

tarsum: TarSum should be the interface
integ-cli: fix TestImportDisplay & add FileServer

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)