rules/control-structᵒ: Treat unquote-splice specially when handling ifs

When making suggestions about ifs, that collapse them into when or
unless, make sure that the body is not unquote-splice-based, because
that can expand to both branches, in which case the suggestion is wrong.

This addresses one half of #9.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
rules/control-structᵒ: Code cleanup

Fold the if=>when & if-not=>unless rules together, to reduce duplicated
code.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>