This is part 1 of a series of proposals to fix issue 4369.

In resolving issue 3507 it was decided not to nil out the inner conn.fd field to avoid a race. This implies the checks for fd == nil inside incref/decref are never true.

Removing this logic removes one source of errClosing error values, which affects issue 4373 and moves towards bradfitz's request that fd.accept() return io.EOF when closed concurrently.

Update #4369.
Update #4373.

R=mikioh.mikioh, bradfitz, dvyukov, rsc
CC=golang-dev
https://golang.org/cl/6852057