channels: Warn when pulling from a mirror.

* guix/channels.scm (<channel-metadata>)[url]: New field.
(read-channel-metadata): Initialize it.
(read-channel-metadata-from-source): Likewise.
(channel-instance-primary-url): New procedure.
(latest-channel-instances): Compare CHANNEL's URL against it.
* doc/guix.texi (Channels)[Primary URL]: New subsection.