optional --writable mount of extfs overlay in --oci mode

[dtrudg]

In --oci mode, when an OCI-SIF containers an overlay (#2867) and --writable is specified on the command line, the overlay should be applied as a read-write overlay, accepting changes made against the container rootfs.

Changes made to the overlay will result in its true content hash changing, and inconsistency vs the image manifest / config. The inconsistency will not be resolved automatically at this time.