Add support for multiple repositories in `emerge --sync`.

Configuration of synchronization is specified by new attributes
supported in repos.conf: sync-cvs-repo, sync-type, sync-uri.

SYNC variable is no longer supported, since it could not override
gentoo.sync-uri attribute now set in default configuration of
repositories (/usr/share/portage/config/repos.conf).
Move hardcoded atoms of cvs, git and rsync to portage.const module and fix category of git atom.