Convert vmstat to use libxo.

This patch was based on this patch:
https://github.com/Juniper/libxo/blob/master/patches/vmstat.patch

by Phil Shafer at Juniper Networks, but updated to the latest
vmstat code.

Reviewed by:           allanjude
Differential Revision: https://reviews.freebsd.org/D3935

usr.bin/vmstat/Makefile

@@ -3,7 +3,7 @@
 
 PROG=	vmstat
 MAN=	vmstat.8
-LIBADD=	devstat kvm memstat util
+LIBADD=	devstat kvm memstat xo util
 
 WARNS?=	1
 

usr.bin/vmstat/vmstat.8

@@ -28,7 +28,7 @@
 .\"	@(#)vmstat.8	8.1 (Berkeley) 6/6/93
 .\" $FreeBSD$
 .\"
-.Dd August 8, 2014
+.Dd November 19, 2015
 .Dt VMSTAT 8
 .Os
 .Sh NAME
@@ -37,6 +37,7 @@
 .Sh SYNOPSIS
 .Nm
 .\" .Op Fl fimst
+.Op Fl -libxo
 .Op Fl afHhimoPsz
 .Op Fl M Ar core Op Fl N Ar system
 .Op Fl c Ar count
@@ -68,6 +69,13 @@ the default image).
 .Pp
 The options are as follows:
 .Bl -tag -width indent
+.It Fl -libxo
+Generate output via
+.Xr libxo 3
+in a selection of different human and machine readable formats.
+See
+.Xr xo_parse_args 3
+for details on command line arguments.
 .It Fl a
 When used with
 .Fl i ,
@@ -361,6 +369,8 @@ statistics every second.
 .Xr ps 1 ,
 .Xr systat 1 ,
 .Xr libmemstat 3 ,
+.Xr libxo 3 ,
+.Xr xo_parse_args 3 ,
 .Xr gstat 8 ,
 .Xr iostat 8 ,
 .Xr pstat 8 ,