accept(2): Update portability note for accept4().

The accept(2) man page warns that O_NONBLOCK and other properties on the
new socket may vary across implementations. However, this issue only
applies to accept() and not to accept4(). On the other hand, accept4()
is not commonly available yet.

Reported by:	pluknet
Reviewed by:	bjk
Approved by:	re (kib)

lib/libc/sys/accept.2

@@ -28,7 +28,7 @@
 .\"     @(#)accept.2	8.2 (Berkeley) 12/11/93
 .\" $FreeBSD$
 .\"
-.Dd May 1, 2013
+.Dd October 1, 2013
 .Dt ACCEPT 2
 .Os
 .Sh NAME
@@ -155,13 +155,20 @@ For some applications, performance may be enhanced by using an
 .Xr accept_filter 9
 to pre-process incoming connections.
 .Pp
-Portable programs should not rely on the
+When using
+.Fn accept ,
+portable programs should not rely on the
 .Dv O_NONBLOCK
 and
 .Dv O_ASYNC
 properties and the signal destination being inherited,
 but should set them explicitly using
-.Xr fcntl 2 .
+.Xr fcntl 2 ;
+.Fn accept4
+sets these properties consistently,
+but may not be fully portable across
+.Ux
+platforms.
 .Sh RETURN VALUES
 These calls return \-1 on error.
 If they succeed, they return a non-negative