[core.counter] Make the value parameter to add default to 1.

src/core/counter.lua

@@ -31,7 +31,7 @@ local counter_t = ffi.typeof("struct counter")
 
 function open (name, readonly) return shm.map(name, counter_t, readonly) end
 function set  (counter, value) counter.c = value                         end
-function add  (counter, value) counter.c = counter.c + value             end
+function add  (counter, value) counter.c = counter.c + (value or 1)      end
 function read (counter)        return counter.c                          end
 
 function selftest ()