Partially revert r255486, the first argument to socketpair() is a socket

domain, not a file descriptor.  Use 'domain' instead of the original 'd'
for this argument to match socket(2).

PR:		199491
Reported by:	sp55aa@qq.com
MFC after:	1 week

lib/libc/sys/socketpair.2

@@ -28,7 +28,7 @@
 .\"     @(#)socketpair.2	8.1 (Berkeley) 6/4/93
 .\" $FreeBSD$
 .\"
-.Dd September 11, 2013
+.Dd May 4, 2015
 .Dt SOCKETPAIR 2
 .Os
 .Sh NAME
@@ -40,13 +40,13 @@
 .In sys/types.h
 .In sys/socket.h
 .Ft int
-.Fn socketpair "int fd" "int type" "int protocol" "int *sv"
+.Fn socketpair "int domain" "int type" "int protocol" "int *sv"
 .Sh DESCRIPTION
 The
 .Fn socketpair
 system call creates an unnamed pair of connected sockets in
-the specified domain
-.Fa fd ,
+the specified communications
+.Fa domain ,
 of the specified
 .Fa type ,
 and using the optionally specified