(the current version UNIX-specific) anyway, so it's useless to try splitting it
to sysdep and generic part. Instead of this, configure script decides (based on
system type and user's wish) what (if any) client should be built and what
autoconfiguration it requires. Also, the client provides its own die/bug/...
functions.
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().
