Don't allow relative -walletdir paths

Also warn if bitcoind is configured to use a relative -datadir path.

Specifying paths relative to the current working directory in a daemon process
can be dangerous, because files can fail to be located even if the
configuration doesn't change, but the daemon is started up differently.

Specifying a relative -datadir now adds a warning to the debug log. It would
not be backwards-compatible to forbid relative -datadir paths entirely, and it
could also be also inconvenient for command line testing.

Specifying a relative -walletdir now results in a startup error. But since the
-walletdir option is new in 0.16.0, there should be no compatibility issues.
Another reason not to use working directory paths for -walletdir specifically
is that the default -walletdir is a "wallets" subdirectory inside the datadir,
so it could be surprising that setting -walletdir manually would choose a
directory rooted in a completely different location.

src/wallet/walletutil.cpp

@@ -9,7 +9,7 @@ fs::path GetWalletDir()
     fs::path path;
 
     if (gArgs.IsArgSet("-walletdir")) {
-        path = fs::system_complete(gArgs.GetArg("-walletdir", ""));
+        path = gArgs.GetArg("-walletdir", "");
         if (!fs::is_directory(path)) {
             // If the path specified doesn't exist, we return the deliberately
             // invalid empty string.