prepare: Add backwards compatibility mode for DASH wallets.

testing notes:
gist.github.com/tecnovert/627f67b7d04746f79c3e9e975458139e

bin/basicswap_prepare.py

@@ -1256,6 +1256,7 @@ def printHelp():
     print('--keysdirpath            Speed up tests by preloading all PGP keys in directory.')
     print('--noreleasesizecheck     If unset the size of existing core release files will be compared to their size at their download url.')
     print('--redownloadreleases     If set core release files will be redownloaded.')
+    print('--dashv20compatible      Generate the same DASH wallet seed as for DASH v20 - Use only when importing an existing seed.')
 
     active_coins = []
     for coin_name in known_coins.keys():
@@ -1588,6 +1589,9 @@ def main():
         if name == 'initwalletsonly':
             initwalletsonly = True
             continue
+        if name == 'dashv20compatible':
+            extra_opts['dash_v20_compatible'] = True
+            continue
         if len(s) == 2:
             if name == 'datadir':
                 data_dir = os.path.expanduser(s[1].strip('"'))
@@ -1933,6 +1937,9 @@ def main():
     chainclients['monero']['walletsdir'] = os.getenv('XMR_WALLETS_DIR', chainclients['monero']['datadir'])
     chainclients['wownero']['walletsdir'] = os.getenv('WOW_WALLETS_DIR', chainclients['wownero']['datadir'])
 
+    if extra_opts.get('dash_v20_compatible', False):
+        chainclients['dash']['wallet_v20_compatible'] = True
+
     if initwalletsonly:
         logger.info('Initialising wallets')
         settings = load_config(config_path)