cleaned up VLS support

[Ben Pfaff]

I happened to be looking at my pspp-cvs folder just when your
commit happened.  Thanks for reworking that.  It looks much
better.  I do have a couple of additional comments:

        * Now width_to_bytes() is only used in the
          sys-file-reader/writer, could it be moved into one of
          those files (or create a new one for common code)?

        * compare_var_shortnames() and hash_var_shortname() are
          complex, but I don't understand why they can't just
          call, respectively, strcmp() and hsh_hash_string().  Is
          something tricky going on?

-- 
"Implementation details are beyond the scope of the Java virtual
 machine specification.  One should not assume that every virtual
 machine implementation contains a giant squid."
--"Mr. Bunny's Big Cup o' Java"