Previously "loyal" got a zero upkeep by virtual of evaluating to zero...

...when casted to an int. I put in an explicit test for this.  On a
related note I updated the wiki to say loyal units are free, removed
the description of "free" and added a note that you could put an
integer there to set a specific upkeep cost.

src/unit.cpp

@@ -1813,6 +1813,9 @@ int unit::upkeep() const
 	if(cfg_["upkeep"] == "full") {
 		return level();
 	}
+	if(cfg_["upkeep"] == "loyal") {
+		return 0;
+	}
 	return lexical_cast_default<int>(cfg_["upkeep"]);
 }