KeywordsManager: support ~* in KEYWORDS

KeywordsManager: support ~* in KEYWORDS

This allows ~* in KEYWORDS to be match by any unstable keyword in the
user's configuration, similar to how * in KEYWORDS already matches any
user configuration. Thanks to Daniel Robbins <drobbins@funtoo.org> for
the suggestion.

Also, the warning about * or -* in KEYWORDS is now gone, so that
ebuilds from the funtoo tree will be usable without triggering
warnings like this. If necessary, we can add ways to selectively
enable these kinds of warnings via layout.conf and/or repos.conf.