Allow stubby to read a file of cmdline options

[raharper]

I would like to be able to place a file of cmdline options for stubby to read from instead of the baked-in values.
The other advantage here is that if stubby is booted via PXE or HTTP, then when the read of the file occurs, this would result in a fetch from the source server of the value allowing us to Network Boot a stubby kernel and pass options to it since we cannot append command line args without adding some sort of interposer (like grub, or ipxe.efi).