RFC: geli+gmirror problem + solution

[Guido van Rooij]

We have a setup where we have two disks in a gmirror with GELI on top
of it. We boot from it (using a small unencrypted USB stick).
We this have the G_ELI_FLAG_BOOT set.

Anyway, when rebooting, geli does not detach and this the gmirror is
never destroyed resulting in a rebuild each time.

I just commited a fix for this (set the G_ELI_FLAG_WO_DETACH in g_eli_taste().
However, this seems to break zfs as zfs might actually close the geli device
and later opening it (but it is then no longer there).

We must therefor make this configurable.

I thus propose to be able to set the G_ELI_FLAG_WO_DETACH in the geli-
metadata, just like the G_ELI_FLAG_BOOT flag. This would mean an
extra option to geli init, say the -d flag.

Any objections?

-Guido