Improve TUN device check

While it looks hackish, I don't think there's a better way (in Bash) to open
the /dev/net/tun character device.

Checking for presence of /dev/net/tun like were doing is not good enough.

openvpn-install.sh

@@ -57,9 +57,11 @@ This version of CentOS is too old and unsupported"
 	exit
 fi
 
-if [[ ! -e /dev/net/tun ]]; then
-	echo "The TUN device is not available
-You need to enable TUN before running this script"
+if [[ -e /dev/net/tun ]] && exec 2>/dev/null 7<>/dev/net/tun; then
+	exec 7>&-
+else
+	echo "This system does not have the TUN device available
+TUN needs to be enabled before running this installer"
 	exit
 fi