`std::net::tcp` docs: Use current syntax and types

Doesn't touch non-comment lines. This changes various type_names to TypeNames
and fixes the example for `tcp::accept` that was still using the old
`match` syntax and `{|args| ...}` closures.
auto merge of #5185 : ben0x539/rust/net-tcp-docs, r=brson

This changes various type_names to TypeNames and fixes the example for `tcp::accept` that was still using the old `match` syntax and `{|args| ...}` closures.

The `accept` example was fairly outdated. I was just going to stay away from all the IO things until the scheduler revamp lands, but `accept` is probably one of the obvious starting points for networking stuff for a learner, and I don't want to get in the way of anyone's enthusiasm.

Doesn't touch non-comment lines, so I hope I will get away without learning about unit tests. It doesn't seem like the test system is set up to extract tests from doc comments right now.
Change the default level of unused_imports to warn
Remove unused imports throughout src/
Add a special FIXME for subduing warnings in rpath.rs
Adding missing imports for tests, and gate off others