Disable priv in std::comm::Port, etc

It's useful to allow users to get at the internal std::rc::comm::Port,
and other such fields, since they implement important traits like
Select.
auto merge of #10562 : ongardie/rust/master, r=brson

It's useful to allow users to get at the internal std::rc::comm::Port,
and other such fields, since they implement important traits like
Select.

See [rust-dev] "select on std::comm::Port and different types" at https://mail.mozilla.org/pipermail/rust-dev/2013-November/006735.html for background.