go.crypto/ssh/terminal: add support for BSD variants

LGTM=agl
R=golang-codereviews, agl
CC=golang-codereviews
https://golang.org/cl/97850043
diff --git a/util.go b/util.go
index 8df94f5..0763c9a 100644
--- a/util.go
+++ b/util.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build linux,!appengine darwin
+// +build darwin dragonfly freebsd linux,!appengine netbsd openbsd
 
 // Package terminal provides support functions for dealing with terminals, as
 // commonly found on UNIX systems.
diff --git a/util_bsd.go b/util_bsd.go
index 1654453..9c1ffd1 100644
--- a/util_bsd.go
+++ b/util_bsd.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build darwin
+// +build darwin dragonfly freebsd netbsd openbsd
 
 package terminal