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.

src/lib/common/nat.c

@@ -104,6 +104,16 @@ getrel(struct natstr *np, natid them)
     return np->nat_relate[them];
 }
 
+/*
+ * Return relations US has with THEM.
+ * Countries are considered allied to themselves.
+ */
+int
+relations_with(natid us, natid them)
+{
+    return us == them ? ALLIED : getrel(getnatp(us), them);
+}
+
 int
 getrejects(natid them, struct natstr *np)
 {