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

src/main.cpp

@@ -1845,8 +1845,8 @@ bool CheckDiskSpace(uint64 nAdditionalBytes)
 {
     uint64 nFreeBytesAvailable = filesystem::space(GetDataDir()).available;
 
-    // Check for 15MB because database could create another 10MB log file at any time
-    if (nFreeBytesAvailable < (uint64)15000000 + nAdditionalBytes)
+    // Check for nMinDiskSpace bytes (currently 50MB)
+    if (nFreeBytesAvailable < nMinDiskSpace + nAdditionalBytes)
     {
         fShutdown = true;
         string strMessage = _("Warning: Disk space is low");

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;