[patch] nextboot(8) arbitrary kernel environment

I have a patch to allow nextboot(8) to set arbitrary kernel environment
variables (not just the kernel dir and kernel_options).  The usage becomes:

Usage: nextboot [-e variable=value] [-f] [-k kernel] [-o options]
       nextboot -D

and the new option is documented as:

     -e variable=value
                 This option adds the provided variable and value to the ker-
                 nel environment.  The value is quoted when written to the
                 nextboot configuration.

The patch also makes -k an option (no longer mandatory).  The patch is at
http://people.freebsd.org/~emaste/nextboot.diff .  I'll commit it in a few
days if no concerns are raised by review or my testing.

-Ed