Fix a problem when using the left key to change the ring and checkbox menu

items. The old code used modulus which does not give the required result,
for C (ISO 1990) and before the result is implementation defined so may have
worked for some compilers / targets but from C (ISO 1999) onwards the sign
of the result comes from the dividend so will not work. (By M. T. Jones)

ChangeLog

@@ -6,6 +6,7 @@ Key:
      * Something changed / fixed
 
 v0.5dev (ongoing development)
+ * Fix using the left key to change the ring and checkbox menu items
  * sed1520: Add an option for inverted segment mapping (idea by R. Buchert)
 
 v0.5.6