experimentally add mallopt tweaks for reduced memory consumption

experimentally add mallopt tweaks for reduced memory consumption

function                                             old     new   delta
mallopt                                                -     126    +126
main                                                  91     117     +26
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(add/remove: 1/0 grow/shrink: 1/0 up/down: 152/0)             Total: 152 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>