(...and ASSERT_NOT_REACHED => VERIFY_NOT_REACHED)
Since all of these checks are done in release builds as well,
let's rename them to VERIFY to prevent confusion, as everyone is
used to assertions being compiled out in release.
We can introduce a new ASSERT macro that is specifically for debug
checks, but I'm doing this wholesale conversion first since we've
accumulated thousands of these already, and it's not immediately
obvious which ones are suitable for ASSERT.
Remap the list of atexit handlers as read-only while we're not actively
writing to it. This prevents an attacker from using a memory write
primitive to gain code execution via the atexit list.
This is based on a technique used in OpenBSD. :^)
