core: Check if any init exists before switching root

core: Check if any init exists before switching root

If we switch root and can't execute an init program afterwards, we're
completely stuck as we can't go back to the initramfs to start
emergency.service as it will have been completely removed by the switch
root operation.

To prevent leaving users with a completely undebuggable system, let's
at least check before we switch root whether at least one of the init
programs we might want to execute actually exist, and fail early if
none of them exists.