units/initrd-parse-etc.service: Conflict with emergency.target

units/initrd-parse-etc.service: Conflict with emergency.target

If emergency.target is started while initrd-parse-etc.service/start is queued,
the initrd-parse-etc job did not get canceled. In parallel to the emergency
units, it eventually runs the service, which starts initrd-cleanup.service,
which in turn isolates initrd-switch-root.target. This stops the emergency
units and effectively starts the initrd boot process again, which likely
fails again like the initial attempt. The system is thus stuck in an endless
loop, never really reaching emergency.target.

With this conflict added, starting emergency.target automatically cancels
initrd-parse-etc.service/start, avoiding the loop.