Enabling more security/debugging features of OpenBSD malloc

[waddlesplash]

There are a few we should consider:

• Free junking. We do this in the kernel under KDEBUG_LEVEL=2 (nightlies), so enabling it for libroot also probably makes some sense.

• Random bytes for heap randomization. I disabled this because it seemed too much of a performance hit at the time. We should retest and at least re-enable the basic initialization, at least.

• Chunk canaries.