units: add missing ordering deps for Conflicts= of emergency.service

1. If we exited emergency mode immediately, we don't want to have an
   irreversible stop job still running for syslog.socket.  I _suspect_ that
   can't happen, but let's not waste effort working out exactly why it's
   impossible and not just very improbable.

2. Similarly, it seems undesirable to have rescue.service and
   emergency.service both running with an open FD of /dev/console, for
   however short a period.