Fixes https://github.com/docker/distribution/issues/1062
This relaxes the naming restrictions places on Docker images to permit
valid hostnames according to [RFC-2396](https://www.ietf.org/rfc/rfc2396.txt).
It deviates from the RFC in the following ways:
1) Allow underscores where we allow hyphens (hostnames don't allow
underscores, which we must for backwards compatibility).
2) Leave "top-level" name segments unrestricted (domains require an
alpha character to begin a top-level domain, e.g. "com").
3) DO NOT allow a trailing dot, as permitted by FQDNs.
Signed-off-by: Matt Moore <mattmoor@google.com>
The main goal of this changeset is to allow repository name components to
consist of a single character. The number of components allowed and the slash
separation requirements have also been clarified.
To go along with this simplification, errant constants and unneeded error types
have been removed.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
