If an MBR partition start or end can be represented by CHS, set

If an MBR partition start or end can be represented by CHS, set
both CHS and LBA values in the MBR partition.

Restores pre-7.0 initialization of MBR partition start/end, using
slightly less opaque code that retains the slightly different
initialization required by GPT protective MBR's.

Fixes booting from disks >8G on systems where the BIOS uses CHS.

Encountered by Paul de Weerd on his Alix.2 using BIOS 0.99.

Much diagnosis and testing by Paul of various iterations. Thanks!