net: consolidate sockaddrToAddr functions

This CL removes sockaddrToAddr functions from socket creation
operations to avoid the bug like issue 7183.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/105100046
diff --git a/src/pkg/net/fd_unix.go b/src/pkg/net/fd_unix.go
index e22861a..7fa43f6 100644
--- a/src/pkg/net/fd_unix.go
+++ b/src/pkg/net/fd_unix.go
@@ -401,7 +401,7 @@
 	return
 }
 
-func (fd *netFD) accept(toAddr func(syscall.Sockaddr) Addr) (netfd *netFD, err error) {
+func (fd *netFD) accept() (netfd *netFD, err error) {
 	if err := fd.readLock(); err != nil {
 		return nil, err
 	}
@@ -438,7 +438,7 @@
 		return nil, err
 	}
 	lsa, _ := syscall.Getsockname(netfd.sysfd)
-	netfd.setAddr(toAddr(lsa), toAddr(rsa))
+	netfd.setAddr(netfd.addrFunc()(lsa), netfd.addrFunc()(rsa))
 	return netfd, nil
 }