Modify the generate_series() table-valued functions so that its first

Modify the generate_series() table-valued functions so that its first
argument (the START value) is required.  Throw an error if that argument
is not supplied.  In this was the series.c loadable extension can be used
as a demonstration of how to code an xBestIndex function to require certain
parameters.  Compile with -DZERO_ARGUMENT_GENERATE_SERIES to obtain the
legacy behavior.

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