Correct units for "-dbcache" and "-prune"

All dbcache-related values in the code are measured in MiB (not in
megabytes, MB) or in bytes.
The GUI "-prune" values in GB are translated to the node values in MiB
correctly. The maximum of the "-prune" QSpinBox is not limited by the
default value of 99 (GB).
Also, this improves log readability.

src/qt/intro.cpp

@@ -10,6 +10,7 @@
 #include <qt/intro.h>
 #include <qt/forms/ui_intro.h>
 
+#include <qt/guiconstants.h>
 #include <qt/guiutil.h>
 
 #include <interfaces/node.h>
@@ -21,7 +22,6 @@
 
 #include <cmath>
 
-static const uint64_t GB_BYTES = 1000000000LL;
 /* Total required space (in GB) depending on user choice (prune, not prune) */
 static uint64_t requiredSpace;