some variables not referenced in another translation unit made static

Also adds some cross-refence comments for some variables that are
referenced in another translation unit.

src/shknam.c

@@ -201,6 +201,7 @@ static const char *const shkhealthfoods[] = {
  * *_CLASS enum value) or a specific object enum value.
  * In the latter case, prepend it with a unary minus so the code can know
  * (by testing the sign) whether to use mkobj() or mksobj().
+ * shtypes[] is externally referenced from mkroom.c, mon.c and shk.c. 
  */
 const struct shclass shtypes[] = {
     { "general store",