Fix file specification, zap partition bits, rectify disk naming errors

Fix file specification, zap partition bits, rectify disk naming errors

In boot_macppc(8), the current format a) claims to support specific disk/MBR
partitions which ofwboot clearly does not parse as such and b) lacks markup
to make clear which optional parts can (not) be omitted in what way.

Fixing both turns
	promdev:partition/filename options
into
	[[promdev:]filename] [-acds]

Same goes for boot(8/macppc), which has been apparently has been copied
from boot(8/amd64) without accounting for all platform specific details.

On amd64, biosboot(8) sees disks as 'sd' which (accidentially?) matches the
kernel driver's sd(4) name;  it also supports specific disklabel(5) slices,
so amd64 can do 'sd0a:/bsd'.

On macppc, disks show up as 'ide', 'cd' or 'hd' which stems from
OpenFirmware alone, not matching the wd(4) driver's name.  Also, ofwboot
always boots off the 'a' label, so macppc can only do 'hd:/bsd'.

Found while installing OpenBSD/macppc from CD inside QEMU but failing to
boot from disk inside QEMU and consulting our manual pages for help wrt.
specifying a boot device.  Turns out QEMU's OpenBIOS simply cannot boot from
MSDOS filesystems (thanks gkoehler), so it wouldn't work no matter what
boot file specification I'd use.

tests/agreement gkoehler
Feedback OK miod
OK tobhe
OK jmc on a previous boot_macppc.8 diff