floppy.cpp: Update class variables to the current encouraged naming convention.

[mgarlanger]

Updated all member variables to be prefixed with m_, the existing code had a few with the prefix but most without. This made it more difficult to follow the code.

[mgarlanger]

@galibert Can you review this? The file has a mix of class members with and without the m_ prefix. The floppy_image_device mainly has it without it, but has m_fs, m_fs_managers, m_make_sound, m_sound_out, m_flux_per_pixel_infos, m_flux_per_combined_track_infos, and m_flux_screen. Also most of the rest of the file is using the prefix.