Fix charset handling for --key-password on command line

It was always converting to UTF-8 for input from the terminal; there
was a plausible reason for using the legacy charset as-is but it's
better to be consistent.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>

main.c

@@ -1307,7 +1307,7 @@ int main(int argc, char **argv)
 			}
 			break;
 		case 'p':
-			vpninfo->cert_password = strdup(config_arg);
+			vpninfo->cert_password = dup_config_arg();
 			break;
 		case 'P':
 			proxy = keep_config_arg();