RFC - two more pending issues for boot0

It has been brought to my attention that there are two more
pending issues for the boot0 code:

1. allow booting from 'DOS Extended' partitions (type 0x5 and 0xf)
   These partition types are currently in the 'kill list' with no
   option to override them (other than poking into the bootsector).

   According to http://www.freebsd.org/cgi/query-pr.cgi?pr=70531
   LILO and GRUB _can_ be installed in those partitions so it
   would make sense to allow them. The change is pretty trivial and safe
   (boot0 already allows booting from any unknown partition) and
   one can disable a particular partition using boot0cfg -m

   This is a trivial change (actually a simplification of the code),
   so I think it should go in unconditionally.

2. preserve the 'NT Disk UID', claimed to be used in Vista as well.

   see http://www.freebsd.org/cgi/query-pr.cgi?pr=127764

   This change, even though trivial in terms of code, is a bit more
   intrusive as it requires to move the data area in boot0, and
   as a consequence requires a patch (though a trivial one) to
   boot0cfg to let it recognise and manipulate the boot sector.

   I have implemented this (in a form slightly different from the PR),
   and it can be conditionally compiled with -DNT_SERIAL ,
   I plan to commit it, but not make it the default.

   I would like to know how useful people consider this feature.

cheers
luigi