implement error_on_unknown_config_opts, slightly improve related code

Signed-off-by: June Strawberry <june@vern.cc>

tuwunel-example.toml

@@ -89,6 +89,14 @@
 #
 #unix_socket_perms = 660
 
+# Error on startup if any config option specified is unknown to Tuwunel.
+#
+# This is false by default to allow easier deprecation or removal of
+# config options in the future without breaking existing deployments. The
+# default behaviour is to simply warn on startup.
+#
+#error_on_unknown_config_opts = false
+
 # tuwunel supports online database backups using RocksDB's Backup engine
 # API. To use this, set a database backup path that tuwunel can write
 # to.