Switch testnet3's message bytes to avoid connecting to old nodes.

The new bytes are based on "11" to appeal to Gavin's 11 fetish.

This breaks existing testnet3 nodes as the blockchain files
are also versioned.  To upgrade a node delete everything
except wallet.dat from your .bitcoin/testnet3 folder.

src/main.cpp

@@ -1978,10 +1978,10 @@ bool LoadBlockIndex(bool fAllowNew)
 {
     if (fTestNet)
     {
-        pchMessageStart[0] = 0xfa;
-        pchMessageStart[1] = 0xbf;
-        pchMessageStart[2] = 0xb5;
-        pchMessageStart[3] = 0xda;
+        pchMessageStart[0] = 0x0b;
+        pchMessageStart[1] = 0x11;
+        pchMessageStart[2] = 0x09;
+        pchMessageStart[3] = 0x07;
         hashGenesisBlock = uint256("000000000933ea01ad0ee984209779baaec3ced90fa3f408719526f8d77f4943");
     }