Be as explicit as possible when describing relation between the -m mask
argument and slices. Adjust example for the same reason.

Allow boot0cfg to force a PXE boot via boot0 on the next boot.
- Fix boot0 to check for PXE when using the pre-set setting for the
  preferred slice.
- Update boot0cfg to use slice 6 to select PXE.  Accept a 'pxe' argument
  instead of a number for the 's' option as a way to select PXE as well.

Revert r209469: it causes the rest of the function to be bypassed.

Approved by:	re (kensmith)

If a device can't be opened, don't keep going but display the error
returned from open() and quit.

PR: bin/81035
Submitted by: Peter Schuller <peter.schuller at infidyne.com>
Approved by: rrs (mentor)

The "write MBR" verb is kept for backward compatibility, but
the DIOCSMBR ioctl has been removed.

in BUGS, as this is a feature.

Bump the date, as it was forgotten in previous commits and the
page has had significant changes recently

boot0.S changes:

+ import a patch from Christoph Mallon to rearrange the various
  print functions and save another couple of bytes;

+ implement the suggestion in PR 70531 to enable booting from
  any valid partition because even the extended partitions that
  were previously in our kill list may contain a valid boot loader.
  This simplifies the code and saves some bytes;

+ followwing up PR 127764, implement conditional code to preserve
  the 'Volume ID' which might be used by other OS (NT, XP, Vista)
  and is located at offset 0x1b8. This requires a relocation of the
  parameter block within the boot sector -- there is no other
  possible workaround.
  To address this, boot0cfg has been updated to handle both
  versions of the boot code;

+ slightly rearrange the strings printed in the menus to make
  the code buildable with all options. Given the tight memory
  budget, this means that with certain options we need to
  shrink or remove certain labels.

and especially:

	make -DVOLUME_LABEL -DPXE the default options.

  This means that the newly built boot0 block will preserve the
  Volume ID, and has the (hidden) option F6 to boot from INT18/PXE.
  I think the extra functionality is well worth the change.

  The most visible difference here is that the 'Default: ' string
  now becomes 'Boot: ' (it can be reverted to the old value
  but then we need to nuke 1/2 partition name or entries to
  make up for the extra room).

boot0cfg changes:

+ modify the code to recognise the new boot0 structure (with the
  relocated options block to make room for the Volume id).

+ add two options, '-i xxxx-xxxx' to set the volume ID, -e c
  to modify the character printed in case of bad input

PR:		127764 70531
Submitted by:	Christoph Mallon (portions)
MFC after:	4 weeks

- While there, make error messages consistent with the rest.

Approved by:	kib (mentor)

  duplication of code in fdisk and boot0cfg. Also make use of g_providername to
  fix an issue with fdisk and boot0cfg not using the correct provider when
  writing the MBR.

Reviewed by:	phk
Approved by:	pjd (mentor)

Pointed out by:	ru