(...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.
Technically, this can be 'exploited' to set the pgid of an exploiting process
to a near-arbitrary new pgid. This can cause conflicts when assigning future pgids,
destroys the session-boundary, and might confuse future pgid-to-session lookups.
In practice, I can't come up with a way that this causes actual harm.
