Use fuse2fs in native mode when 'allow kernel extfs = no'

[dtrudg]

Is your feature request related to a problem? Please describe.

Recent versions of SingularityCE have a singularity.conf directive that permits disabling kernel mounts of extfs, which are performed in the setuid flow. There is no elegant fall-back:

$ singularity run docker://alpine
$ singularity run with-overlay.sif 
FATAL:   container creation failed: while setting overlay session layout: while adding ext3 image: ext3 image mounts are not authorized

Describe the solution you'd like

fuse2fs is widely available, although only very recent versions allow specification of an offset, necessary to use fuse2fs to mount an extfs embedded in a SIF file.

It should be possible for the kernel mount to fall back to a fuse2fs mount in the setuid flow.