Switch hardened derivation marker to h in descriptors

This makes it easier to handle descriptor strings manually. E.g. an RPC call that takes an array of descriptors can now use '["desc": ".../0h/..."]'.

Both markers can still be parsed. The default for new descriptors is changed to h. In normalized form h is also used. For private keys the chosen marker is preserved in a round trip.

The hdkeypath field in getaddressinfo is also impacted by this change.

test/functional/wallet_backwards_compatibility.py

@@ -253,7 +253,7 @@ class BackwardsCompatibilityTest(BitcoinTestFramework):
         wallet = node_v17.get_wallet_rpc("u1_v17")
         address = wallet.getnewaddress("bech32")
         v17_info = wallet.getaddressinfo(address)
-        hdkeypath = v17_info["hdkeypath"]
+        hdkeypath = v17_info["hdkeypath"].replace("'", "h")
         pubkey = v17_info["pubkey"]
 
         if self.is_bdb_compiled():