handle some common reparent cases.

Screwing around with setns and friends can cause our ppid to change.
We don't handle this well at all, so we'll have to just exit the child
for now until we come up with a better plan.
output whether or not main is still alive when we get reparented
remove the 'dump previous syscalls' from check_parent_pid

we only call this periodically, so the last syscalls are likely irrelevant.
For the same reason, remove the TODO about postmortem