bug#10877: Wimpy external files.

[Pádraig Brady]

A general note on this is to be wary of cache levels.
It may be more efficient to sort in chunks of the
cache level of the machine rather than as large
a chunk as possible.

Some very quick testing here (a while ago),
shows -S2M -T/dev/shm was as fast or faster than
big buffers, as well as being "nicer" to the rest
of the system.

cheers,
Pádraig.

p.s. ensure MALLOC_PERTURB_ is unset when benchmarking sort