sys-file-writer: Write identical sets of value labels only once.

The .sav file format can save space by writing a single set of
value labels once and assigning it to multiple variables.  PSPP
has never taken advantage of this, instead writing the value
labels for each variable separately.  This commit adopts the
strategy for avoiding duplicates apparently used by SPSS, based
on observing a .sav file that had many groups of variables with
the same labels.

Thanks to Curt Reinhold <Curt.Reinhold@psychonomics.de> for
providing the .sav file that led to this discovery.