convert should set the output file read-only

[boredzo]

The intended usage pattern for conversion is to treat the HFS+ volume as a working copy of the HFS volume that happens to be mountable for data retrieval/recovery purposes, not to put it into day-to-day usage. To discourage the latter, we should set the output volume as read-only, so that (unless the user sets it read/write explicitly by their own action) the volume can only be retrieved from, not stored to.

Moreover, even mounting a volume normally changes it (at least its last-mounted date, and potentially rebuilds the desktop on classic Mac OS); a read-only image prevents this.