Skip softraid(4) keydisks

Skip softraid(4) keydisks

Keydisks appear as chunks internally (with special properties) and
installboot(8) thus treated them like actual data chunks.

Most users probably don't hit this as their keydisk is detached and thus
appears "offline" and gets skipped.

Installing to online keydisks may work but is neither expected nor intended
to work, so properly skip them.

Odd setups like keydisk and CRYPTO chunk on the same physical disk would
end up installing getting bootblocks installed twice.

Pointed out by Mikolaj Kucharski <mikolaj AT kucharski DOT name> who also
provided the actual diff (minor wording tweaks by me)

OK jsing