Save last mounted image file on switchable devices

[nielsmh]

For emulated device types that allow switching the mounted disk image during operation (right now CD-ROM, in the future perhaps also floppy, ZIP and tape) it could be useful with an option to have the last mounted image be re-mounted on next startup.

The FlashFloppy project does this by storing a file on the attached USB device, that contains the filename of the last mounted image.

A feature like this should probably also have a way to either turn it off, or require it to be manually enabled. The reason being that in some situations you might need to always start with the same image mounted, for example a specific boot floppy. One way could be to only enable the feature if the file that stores the last mounted image is present on the SD card, and otherwise disable it.

An idea for a filename could be lastimage-CD1.txt. Start it with a string that does not resemble a device name, but include the device name it pertains to in the filename. Name it as .txt to indicate that this is a user-readable file, and the user might also be able to edit it.
Be lenient when reading the file, accept that it might contain newlines and such, and that the image filename it indicates might be misspelled or gone.