fix off-by-one in lookahead code

The Lookahead var should indicate how often the counter is incremented
before an offer is finally declared invalid, *not* the total number of
checks (aka window size).
I.e. for Lookahead=0, check only against the current counter instead of
not at all.

goatherd.go

@@ -190,7 +190,7 @@ func check_offer(db *sql.DB, remote string, name string, offer string) (bool, er
             return false, err
         } else { err_panic(err) }
 
-        for i := uint64(0); i < cfg.Lookahead; i++ {
+        for i := uint64(0); i <= cfg.Lookahead; i++ {
             debugf("[%v] checking for match (offset %v)", remote, i)
             // .Check increments .Counter if successfull
             // otherwise do it explicitly