necessary because CLOOP format lacks explicit EOF or length, so that
in the presence of padding or when the CLOOP is put onto a larger
partition upper level provider size may be larger. Bound amount
of extra data that we might touch to the max length of the compressed
block and detect zero-padding in the last cluster, which when
sector is all-zero might cause us to emit bogus I/O error after
decompression of that fails. To not make code any more complicated
that it needs to be deal with it in lazy-manner, i.e. when we
first access that specific cluster.

This change also fixes stupid mistake in the LZMA code, inherited
from geom_lzma, which does not share length of the output buffer
buffer with the decompression routine, so that in the presence
of corrupted or purposedly tailored data may easily cause heap
overflow and kernel memory corruption.

Beef up validation of the CLOOP TOC by checking that lengths of
all but the last compressed clusters match upper limit set by
the decompressor and improve some error diagnostic output while
I am here.

attaching uzip to certain devices in the dev tree only.

  For example the following only makes us attaching to the
  GPT labels:

  kern.geom.uzip.attach_to="gpt/*"

above, i.e. prevents geom_uzip from tasting / attaching to
providers matching some pattern. By default we don't attach
to our own kind, i.e. kern.geom.uzip.noattach_to="*.uzip".
It saves us quite some CPU cycles, esp on low-end embedded
systems.