Add (optional) ignoring of anything with a `_` prefix

[Pomax]

In order to offer users an additional mechanism to prevent remote calls to local functions that should stay inaccessible, it would be useful to add an optional disallow accessing anything that starts with an underscore. This should be enabled by default, but configurable by the user.

This can probably be done by either adding logic to the SocketProxy (in upgraded-socket), when it's deciding whether or not to allow a call to go through over the network or not, or adding logic to the route code (in the same file) that throws when a callable is a locked property.