[rpc] getblockchaininfo: add size_on_disk, prune_target_size, automatic_pruning

Fix pruneheight help text.
Move fPruneMode block to match output ordering with help text.
Add functional tests for new fields in getblockchaininfo.

src/validation.h

@@ -280,6 +280,9 @@ CAmount GetBlockSubsidy(int nHeight, const Consensus::Params& consensusParams);
 /** Guess verification progress (as a fraction between 0.0=genesis and 1.0=current tip). */
 double GuessVerificationProgress(const ChainTxData& data, CBlockIndex* pindex);
 
+/** Calculate the amount of disk space the block & undo files currently use */
+uint64_t CalculateCurrentUsage();
+
 /**
  *  Mark one block file as pruned.
  */