make CheckDiskSpace() use a global static const uint64 for checking required disk space and remove 2 ugly spaces from a message string

src/main.h

@@ -75,8 +75,8 @@ extern unsigned char pchMessageStart[4];
 // Settings
 extern int64 nTransactionFee;
 
-
-
+// Minimum disk space required - used in CheckDiskSpace()
+static const uint64 nMinDiskSpace = 52428800;
 
 
 class CReserveKey;