[slx-dmsetup] Avoid unnecessary dm targets, rename crypted ones

1)
Avoid creating linear targets that would be fully covered by an
encrypted target later. Also don't create a linear slice for the
thinpool if it's going to be the only thing we put in our writable
partition.

2)
Previously, when we created an encrypted device, it would look like

 |
 ` devicename
    ` devicename-crypt

Which was a bit confusing/surprising, because the final name was not
what was specified in our custom partition table. Now, the result would
look like

 |
 ` devicename-k
    ` devicename

So the final device you'd use has the expected name.

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