These a series of changes fix inconsistent errors on the package net
APIs. Now almost all the APIs return OpError as a common error type
except Lookup, Resolve and Parse APIs. The Lookup, Resolve and Parse
APIs return more specific errors such as DNSError, AddrError or
ParseError.

An OpError may contain nested error information. For example, Dial may
return an OpError containing a DNSError, AddrError, unexposed type/value
or other package's type/value like the following:
	OpError{/* dial info */, Err: &DNSError{}}
	OpError{/* dial info */, Err: &AddrError{}}
	OpError{/* dial info */, Err: <unexposed type or value>}
	OpError{/* dial info */, Err: <other package's type or value>}

and Read and Write may return an OpError containing other OpError when
an application uses io.Copy or similar:
	OpError{/* for io.Reader */, Err: &OpError{/* for io.Writer */}}

When an endpoint is created for connection-oriented byte-stream
protocols, Read may return an io.EOF when the connection is closed by
remote endpoint.

Fixes #4856.

A series of changes:
- net: fix inconsistent error values on Dial, Listen partially
  https://go.googlesource.com/go/+/89b7c66d0d14462fd7893be4290bdfe5f9063ae1
- net: fix inconsistent error values on Read
  https://go.googlesource.com/go/+/ec1144423f45e010c72363fe59291d43214b6e31
- net: fix inconsistent error values on Write
  https://go.googlesource.com/go/+/11b5f98bf0d5eb8854f735cc332c912725070214
- net: fix inconsistent error values on Close
  https://go.googlesource.com/go/+/310db63c5bc121e7bfccb494c01a6b91a257e7fc
- net: fix inconsistent error values on Accept
  https://go.googlesource.com/go/+/4540e162b1aefda8157372764ad3d290a414ef1d
- net: fix inconsistent error values on File
  https://go.googlesource.com/go/+/885111365ba0a74421059bfbd18f4c57c1e70332
- net: fix inconsistent error values on setters
  https://go.googlesource.com/go/+/2173a27903897c481b0a0daf3ca3e0a0685701db
- net: fix inconsistent error values on Interface
  https://go.googlesource.com/go/+/456cf0f22c93e1a6654980f4a48a564555f6c8a2
- net: fix inconsistent error values on Lookup
  https://go.googlesource.com/go/+/0fc582e87942b2e52bed751b6c56660ba99e9a7d
- net: add Source field to OpError
  https://go.googlesource.com/go/+/afd2d2b6df3ebfe99faf347030f15adfdf422fa0

Change-Id: Id678e369088dc9fbe9073cfe7ff8a8754a57d61f
Reviewed-on: https://go-review.googlesource.com/9236Reviewed-by: Ian Lance Taylor <iant@golang.org>