[Config] Ignore prune mode if txindex is set

[CaveSpectre11]

Problem

#253 - Prune mode incompatible with txindex.
Having prune mode on can stick the user into being unable to start their wallet. For Qt: they receive an error stating "Prune mode is Incompatible with -txindex"

Root Cause

The error was a little heavy handed, and rejected the use of prune mode without txindex=0. The part of this issue that makes it important to address is that you can inadvertently end up with bPrune set in your registry on windows; making it difficult to find and remove. There is an additional issue that implies that uninstalling your wallet will not remove the registry entries; so getting bPrune=true in your registry would be quite challenging to clear out, and likely result in support tickets.

Solution

Note that prune mode is unsupported and likely will result in corrupting your wallet. This is not meant to claim that prune mode is functional.

If Prune mode is set and txindex is not implicitly disabled [txindex=0], the wallet will not run. This PR changes the prune mode from an error that shuts down the wallet, to a warning that ignores prune mode if txindex is enabled. This warning will not show in a Windows pop-up, but rather in the log file and the terminal executing the daemon and/or qt. It will also ignore the prune mode setting, changing it to '0' if it is set.

Testing

combinations of txindex=[0|1], prune=[0|1] in your config file, command line, and bPrune=true in your registry [see issue #253 for registry details]

[codeofalltrades]

ACK 5d49651

Set the prune setting via the GUI.
tested with
txindex=1, prune=1
txindex=0, prune=1
txindex=1, prune=0
txindex=0, prune=0

All the test passed.
OS Win10