Today, because we don't wait for execvpe to finish to continue onwards,
it's possible that if you did an exec quick enough after starting an
init process for a container, that you could join the init processes
namespaces before pivot_root has taken place which is quite fun. Let's
wait for exec to finish (or an error to occur) to prevent this.
We should namespace these a bit, and we need pidfd for some upcoming
terminal work. This also just gets rid of syscall2 and replaces with a
pivot_root wrapper.
Reverts f9198d6ff7
We can explore better approaches to some of the targets, but for now
lets revert as it interferes with being able to build even if we have
local changes.
Danny Canter