net: merge common Unix/Windows methods

When we added the internal/poll package, the Unix and Windows implementations
of several netFD methods became exactly the same, except for using a
different name for the string passed to wrapSyscallError.

One case is not an exact duplicate: we slightly tweak the implementation
of (*netFD).shutdown on Windows to wrap the error.

Change-Id: I3d87a317d5468ff8f1958d86f6189ea1ba697e9a
Reviewed-on: https://go-review.googlesource.com/c/go/+/224140


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>