Remove mbr/*.bin on "spotless", not "clean"; otherwise we break "make
installer".

Fix an invalid segment assumption in pxelinux, which caused the
"naked ::" not to work correctly.

The $PnP probe has been found to lock up at least one machine for
reasons unknown.  Drop it; instead, burn the extra few bytes in the
bootsector to save away ES:DI for later restore.

Fix loading from logical partitions (a push/pop pair was not done in
reverse order.)

Change DriveNo to DriveNumber in ISOLINUX, to match the other
derivatives.

cleanup_hardware has registers live across it when called from the
boot sector code; it must preserve all registers.

Remove unused variable BufSafeSec; it was identical to BufSafe in
every way.

At the moment, BufSafe* are always constants; consider if they can be
swapped out for constants or if we want to retain them for flexibility.

Add support for sha256crypt and sha512crypt to the library.  Not yet
included in the menu system, need to decide if it's worth 10K or if
the code can be realistically compacted.

For 32-bit code, 32-bit operations are shorter than 16-bit operations,
so use them.