VFATLIB - FAT32: Correctly compute the number of free clusters for the FSInfo structure

FAT32: Correctly compute the number of free clusters for the FSInfo structure, and backup the FSInfo sector either (as it's done for the bootsector). The VFATLIB FAT checker and some other tools relied on that.

References:

• https://de.wikipedia.org/wiki/Volume_Boot_Record
• http://thestarman.pcministry.com/asm/mbr/MSWIN41.htm#FSINFO
• "Microsoft Extensible Firmware Initiative - FAT32 File System Specification" https://staff.washington.edu/dittrich/misc/fatgen103.pdf