Filter code used 'aux' integer field of 'symbol' struct to store ptr
to next symbol and both 'aux2' and 'def' fields for value.
Changed to just 'def' for value and 'aux2' for ptr to next symbol.
Also another minor bugfix.
over EFence and also hopefully smaller memory overhead, but sadly it's non-free
for commercial use).
If the DMALLOC_OPTIONS environment variable is not set, switch on `reasonable'
checks by default.
Also introduced mb_allocz() for cleared mb_alloc().
