Mount disk images as read/write by default
I only use GDU to open encrypted drive images. In this context, read/write is the appropriate default for me as I want to edit the files in my disk image. I imagine people would want to mount things read-only to prevent accidental changes, but that would be the minority use-case.
I looked through the history, but couldn't find any rationale in the code for why this was done in the history besides reference to DVDs. In that context, it does make sense to open things on DVD-R and CD-R as read-only. Is that a common usecase for GDU?
This is related to #126 and #80, but those are about the default for when you've just created a new image. I think the argument there is a much clearer usecase.