This commits brings in a lot of documentation and some enhancement

of the boot0.S code, with a number of compile-time selectable options,
the most interesting one being the ability to select PXE booting.

The code is completely compatible with the previous one, and with
the boot0cfg program. Even the actual code is largely unmodified,
with only minor rearrangements or fixes to make room for the new
features.

The behaviour of the standard build differs from the previous
version in the following, minor things:

+ 'noupdate' is the default, which means the code does not
  write back the selection to disk. You can enable the feature
  at runtime with boot0cfg, or changing the flags in the Makefile.

+ a drive number of 0x00 (floppy, or USB in floppy emulation) is
  now accepted as valid. Previously, it was overridden with 0x80,
  meaning that the partition table coming from the media was
  used to access sectors on a possibly different media.
  You can revert to the previous mode building with -DCHECK_DRIVE,
  and you can always use the 'setdrv' option in boot0cfg

+ certain FAT or NTFS partitions are listed as WIN instead of DOS.

+ the 'bel' character on a bad selection is replaced by a '#' to
  make it clear that the system is not hang even if the machine
  does not have a speaker. This can be reverted back at compile
  time, or at runtime with an upcoming boot0cfg option.

Additional features are available as compile time options,
and may be become the default if deemed useful. In particular:

+ INT18/PXE boot (make -DPXE)
  This option enables booting through INT 18h (which on certain
  BIOSes can be hooked to PXE) by pressing F6. There is unfortunately
  no room to print the additional menu option.
  Also, to make room for the code, the 'Default: ' string is
  changed to 'Boot: '

+ print current drive number (make -DTEST)
  Prints a line indicating the current drive number.
  This is useful to figure out what is going on for machines/bioses
  which remap drives in sometimes surprising ways.

+ disable numeric keys in console mode (make -DONLY_F_KEYS)
  Not really a significant option, but it is needed to make
  room for the -DTEST mode.

+ disable floppy support (make -DCHECK_DRIVE)
  Revert to the old behaviour of only accepting 0x80 and above
  as valid drive numbers.

MFC after:	6 weeks

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