Add support for /etc/portage/package.accept_keywords. If both

Add support for /etc/portage/package.accept_keywords. If both
package.accept_keywords and package.keywords are present, both
of them will be used, and values from package.accept_keywords
will override values from package.keywords. The
package.accept_keywords file is intended to replace the
package.keywords file, since profiles support a different form
of package.keywords which modifies effective KEYWORDS (rather
than ACCEPT_KEYWORDS). Note that bug #292083 requests support
for package.accept_keywords in profiles.