add RECALL to core unit macros

Many UMC projects already have a RECALL macro.

data/core/macros/unit-utils.cfg

@@ -165,6 +165,12 @@
     [/unit]
 #enddef
 
+#define RECALL ID_STRING
+    [recall]
+        id={ID_STRING}
+    [/recall]
+#enddef
+
 #define GUARDIAN
     # Meant to be used as a suffix to a unit-generating macro call.
     [+unit]