Implement !GPT protocol in Syslinux core; handle offset > 2 TB

Implement !GPT protocol in Syslinux core; handle offset > 2 TB

Implement the !GPT handover protocol in the Syslinux core, and handle
partition offsets above 2 TB.  We do not yet handle filesystem sizes
above 2 TB, but that should be a reasonably straightforward extension
at this time (need to switch to 8-byte block pointers).

This finally meant moving getlinsec out of the boot sector, since it
no longer fits.  Instead have a very simple getonesec implementation
in the boot sector.  getlinsec still fits in Sector 1, although it
doesn't leave space for very many block pointers.  That's still better
than adding yet another boot loader stage, however.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>