Support for rootless containers

[SvenDowideit]

I'm trying out some things with rootless namespace vs no-namespace container usage, and wonder if @AkihiroSuda 's fork is necessary, or if these changes can be merged

:)

I've basically naively rebased - so feel free to tell me off.

[AkihiroSuda]

thanks

[AkihiroSuda]

I think https://github.com/rootless-containers/PRoot/issues/2 needs to be fixed

[AkihiroSuda]

also needs https://github.com/rootless-containers/PRoot/commit/2a2a37e8e35ff7fabfc2b1faade3959a78cf44ff

[brad-hue]

Thanks