Location proxied rocks get protected

[tseaver]

In https://bugs.launchpad.net/zope.security/+bug/516588, @jinty reported:

ProxyFactory(LocationProxy(rock)) results in a non-accessable object.

And attached a patch with a failing test:

https://bugs.launchpad.net/zope.security/+bug/516588/+attachment/1127429/+files/test_locationproxied_rocks.patch

@tseaver followed up:

I can confirm that the suggested test fails, and that (from my [understanding of the model) it should pass.

The problem arises into the monkey-patch of the DecoratedSecurityCheckerDescriptor as the __Security_checker__ for location proxies. That descriptor insists that either the unwrapped object has a checker, or falls back to the wrapper. However, the case where selectChecker(proxied_object) returns None implies that no security proxy is to be used at all. There are tests which assert the (seemingly incorrect) behavior of honoring the checker on the wrapper.

I'm assigning this to Jim, as I don't think anybody else understands the model well enough to make the call here.