Explicitly prevent use of swap_file resources on FreeBSD

The code will not work on FreeBSD because mkswap is missing and swapon has a
differing syntax from Linux. As it is, the swapfile will be created on FreeBSD
but everything after that fails.

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>
Fix permissions on the swapfile

The generated swapfile had world-readable permissions by default (root:root,
644). Added a File resource which ensures that permissions are set to 600.

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>