service: add restart on-failure to pveproxy and pvedaemon

service: add restart on-failure to pveproxy and pvedaemon

This way they can cleanly exit (with a SIGTERM or systemctl stop) but
will get restarted if killed or they abort, but only for
StartLimitIntervalSec= time period with a total maximal count of
StartLimitBurst= retries.
See `man systemd.unit` for details, default are 10s total
restart retry period with at max 5 total retry counts, after that the
unit will placed in the failure state. So this is a best effort try
with no real downside.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>