FREQUENCIES: Modernize syntax and improve chart support.

The FREQUENCIES syntax was carrying a lot of baggage from old versions
that is no longer mentioned in the SPSS syntax reference.  This commit
removes support for those obsolete features.

The implementation only supported a single chart at a time.  Now it
supports pie charts and histograms in a single command.  The MIN and
MAX format options are now implemented, as are the MISSING/NOMISSING
options for pie charts.  (The FREQ/PERCENT options are still not
implemented.)

Also update documentation to match.