MFC 228738:

MFC 228738:
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.

git-svn-id: svn://svn.freebsd.org/base/stable/9@230065 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f