Check for overlay kernel module

[staticfloat]

If a user has a kernel with CONFIG_OVERLAY_FS=m set, we need to ensure
that the overlay module is actually loaded.

Closes #61

[staticfloat]

@vchuravy can you check to ensure that this actually works on your system? It should give you a warning if your module is not loaded.

[vchuravy]

Thanks! After sudo rmmod overlay

┌ Warning: Could not find loaded `overlay` module, try `sudo modprobe overlay`?
└ @ Sandbox ~/.julia/packages/Sandbox/k7x0e/src/UserNamespaces.jl:114

What if the kernel was built with CONFIG_OVERLAY_FS=Y I don't think it will be listed in the modules, right?

[staticfloat]

I haven't been able to find a machine with that set to try it out with. :P

[staticfloat]

Okay, I checked by building myself a new kernel, and using the VERY COOL virtme project to quickly boot up a shell and see what /proc/modules shows on such a system, which is, as Valentin predicted, exactly nothing. I have therefore built an escape hatch for this (apparently quite unusual) situation, so users who know they can mount overlayfs filesystems can bypass the check.