core: change default scantime/expiry to 7/28 (back to 1:4 ratio).

Previous defaults (before 10/30) were optimised for Bitcoin that has
a much higher average block time. Optimise for Litecoin (currently
largest scrypt network.)

Not pushing it further down so as not to scare people with dropping
hashrates.

Thanks to MissedOutOnBTC @ freenode for testing.

Closes https://github.com/veox/sgminer/issues/22

sgminer.c

@@ -90,8 +90,8 @@ bool opt_compact;
 const int opt_cutofftemp = 95;
 int opt_log_interval = 5;
 int opt_queue = 1;
-int opt_scantime = 10;
+int opt_scantime = 7;
-int opt_expiry = 30;
+int opt_expiry = 28;
 static const bool opt_time = true;
 unsigned long long global_hashrate;
 unsigned long global_quota_gcd = 1;