Use rpm-packaged logdir for munin-node

The rpm uses a different logdir for munin-node:

    [root@server12 ~]# rpm -ql munin-node|grep log/
    /var/log/munin-node

Without this, a fresh build tries to log to `/var/log/munin` and gets in a reboot loop.

Also records PID to a file in `/var/run` to avoid a possible missing dir there.