don't enable backtrace() for stacktrace capturing by default

Because we don't yet have a treatment for deadlocks that are caused by
(recursive) use of malloc from within that facility.
issue-610: introduced TCMallocGetenvSafe

This is version of GetenvBeforeMain that's available to C code.
issue-610: made dynamic_annotations.c use TCMallocGetenvSafe
issue-610: use TCMallocGetenvSafe from inside malloc

Instead of plain getenv. So that windows getenv implementation that
may call malloc does not deadlock.