[FEATURE] Allow mounting SD images in R/O mode

[wavexx]

Is your feature request related to a problem? Please describe.
I'm looking at some test SD images which I made intentionally R/O. These fail to mount, since in SDCard.cpp:571 we explicitly open them with O_RDWR with no fallback.

Describe the solution you'd like
If open() fails, we should attempt to reopen the file with O_RDONLY or probe using access(2).

If O_RDONLY succeeds, for a proper simulation, mmap shouldn't have PROT_WRITE and we should also mark the CSD's PERM_WRITE_PROTECT bit and return errors during writes.

Describe alternatives you've considered
For now I just attempt to reopen with O_RDONLY to ensure I don't change testing images, but that's not a PR I would submit ;)