Split check_flags() off check_keys() and have check_exprs() call it
later, so its error messages gain an "in definition" line. Tweak the
error messages.
Checking values in a function named check_keys() is unclean anyway.
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190927134639.4284-20-armbru@redhat.com>