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.
Backport of Core PR15163 and PR15801
The second PR fixes a bug in the first one ([[https://github.com/bitcoin/bitcoin/pull/15801/commits/4ddeb2f860eee98fbe94725ea8885368068a03f2 | 4ddeb2f860eee98fbe94725ea8885368068a03f2]]) and then extends the prune limit even further.