Change memory-space accounting mechanism in tuplesort.c and tuplestore.c

to make a reasonable attempt at accounting for palloc overhead, not just
the requested size of each memory chunk.  Since in many scenarios this
will make for a significant reduction in the amount of space acquired,
partially compensate by doubling the default value of SORT_MEM to 1Mb.
Per discussion in pgsql-general around 9-Jun-2002..