Discard junk from partition tables when reading the tables for applications

Discard junk from partition tables when reading the tables for applications
the same as when initializing the in-core copies.  Adjust checksums in
labels after adjusting labels.  This finishes fudging the on-disk label to
make it coherent with the in-core label.

Handle EIO during initialization better.

Initialize the compatibility slice to the whole disk If there are no real
slices.

Don't warn about adjusting offsets in the label to make the 'c' partition
start at 0.  The 'c' offset is now always absolute on-disk and 0 in-core
so an adjustment is usually required.

Don't confuse LABEL_PART with RAW_PART so much.

Check for partitions being within slices differently.