At present, the series of File{Conn,Listener,PacketConn} APIs are the
only way to configure platform-specific socket options such as
SO_REUSE{ADDR,PORT}, TCP_FASTOPEN. This change adds missing test cases
that test read and write operations on connections created by File APIs
and removes redundant parameter tests which are already tested in
server_test.go.

Also adds comment on full stack test cases for IPConn.

Fixes #10730.

Change-Id: I67abb083781b602e876f72a6775a593c0f363c38
Reviewed-on: https://go-review.googlesource.com/17476Reviewed-by: Ian Lance Taylor <iant@golang.org>