interface: Don't mutate user options

[RafaGago]

This patch fixes cases where the 'opts' table passed by the user is modified, making the external table unable to be reused in some cases.

This was causing problems e.g. when dealing with properties, as the user was having a predefined dbus options table, as the "interface" key was changed internally to 'org.freedesktop.DBus.Properties' without being changed back on return.