cmd/util_bsd: add BSD operating systems

Even though we don't directly support BSD systems, it doesn't mean we can't
help them to build `lab` if the work is pretty small. This patch adds the
three officialy supported BSD OSes by golang to the cmd/util_bsd file, since
they are pretty close to what we need in lab from darwin (or the other way
around? :).

Signed-off-by: Bruno Meneguele <bmeneg@redhat.com>

cmd/util_bsd.go

@@ -1,6 +1,6 @@
 // This file contains Darwin (MacOS) and *BSD specific calls.
 
-// +build darwin
+// +build freebsd openbsd dragonfly darwin
 
 package cmd