Clean up unix vs linux usage

FreeBSD does not support cgroups or namespaces, which the code suggested, and is not supported
in runc anyway right now. So clean up the file naming to use `_linux` where appropriate.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

libcontainer/configs/cgroup_linux.go

@@ -1,5 +1,3 @@
-// +build linux freebsd
-
 package configs
 
 type FreezerState string

libcontainer/configs/config_linux.go

@@ -1,5 +1,3 @@
-// +build freebsd linux
-
 package configs
 
 import "fmt"

libcontainer/configs/config_linux_test.go

@@ -1,5 +1,3 @@
-// +build linux freebsd
-
 package configs
 
 import (

libcontainer/configs/namespaces_linux.go

@@ -1,5 +1,3 @@
-// +build linux freebsd
-
 package configs
 
 import (

libcontainer/configs/namespaces_unsupported.go

@@ -1,4 +1,4 @@
-// +build !linux,!freebsd
+// +build !linux
 
 package configs
 

libcontainer/criu_opts_linux.go

@@ -1,5 +1,3 @@
-// +build linux freebsd
-
 package libcontainer
 
 // cgroup restoring strategy provided by criu

libcontainer/devices/devices_linux.go

@@ -1,5 +1,3 @@
-// +build linux freebsd
-
 package devices
 
 import (

libcontainer/devices/devices_unsupported.go

@@ -1,3 +1,3 @@
-// +build windows
+// +build !linux
 
 package devices

main_linux.go

@@ -1,5 +1,3 @@
-// +build linux
-
 package main
 
 import (

main_solaris.go

@@ -1,5 +1,3 @@
-// +build solaris
-
 package main
 
 import "github.com/urfave/cli"