New relations_with()

New relations_with()

Relations checking with getrel() often needs a special case for "is
same country".  If you forget, you get behavior appropriate for a
neutral foreign country, which is usually very wrong (see commit
16c68eb4 for an example).

Unlike getrel(), relations_with() considers countries allied to
themselves.  Less dangerous.  In fact, allied behavior is typically
just right, so the special case isn't even needed.