de-duplicate fuse mounting/unmounting code

[preminger]

Description of the Pull Request (PR):

Transition as much FUSE-related code as is currently feasible to using internal/pkg/util/fs/fuse package.

This PR addresses FUSE-related code in the following source files:

• internal/pkg/runtime/launcher/native/launcher_linux.go, and dependencies thereof
• internal/pkg/runtime/engine/singularity/cleanup_host_linux.go, and dependencies thereof

(These two are related to use of the --sif-fuse flag in native mode.)

This PR does not address FUSE-related code in the following source files (see #2201):

• internal/pkg/runtime/engine/singularity/container_linux.go
• internal/pkg/runtime/engine/singularity/prepare_linux.go
• internal/pkg/runtime/engine/singularity/process_linux.go

(These three are related to passing FUSE mounts by file-descriptor from host to container.)

This fixes or addresses the following GitHub issues:

• Fixes chore: de-duplicate fuse mounting/unmounting code #2025

[dtrudg]

Taking shape nicely. I've left some comments. My chief concern is that extra options must not be allowed to potentially override things that are set from specific attributes of ImageMount.

[dtrudg]

This is LGTM - but I think I missed something r.e. uid / gid handling which should probably at this point mean that they are handled as attributes?

[dtrudg]

LGTM - noted some typos in comments