unit: rename BindTo= to BindsTo=

all other dependencies are in 3rd person. Change BindTo= accordingly to
BindsTo=.

Of course, the dependency is widely used, hence we parse the old name
too for compatibility.

src/core/target.c

@@ -59,7 +59,7 @@ static int target_add_default_dependencies(Target *t) {
                 UNIT_REQUISITE,
                 UNIT_REQUISITE_OVERRIDABLE,
                 UNIT_WANTS,
-                UNIT_BIND_TO
+                UNIT_BINDS_TO
         };
 
         Iterator i;