[FREELDR]

- In the fat bootsector, don't clobber ax, it contains the number of first cluster of freeldr.sys and is used by the fathelper code.
- Fix jump code in the fathelper code, like in the other bootsectors.
- Now booting from fat partitions (looks like that's what sysreg does) works again. It's safe under the condition that the cluster size is at least 4352 bytes, which is true for harddisks of sizes bigger than 272MB. Booting from smaller fat disks, like floppy breaks when freeldr.sys gets fragmented, which should rarely happen.

svn path=/trunk/; revision=48177