Wolfgang, which version of Ubuntu are you using? At a fresh install of Ubuntu 12.04 LTS (without the transitional dhcp3-server package installed) the directory /var/run/dhcp3-server/ does not exist.
For you, instead of patching the isc-dhcp-server script,
ln -s /var/run/dhcp3-server/dhcpd.pid /var/run/dhcpd.pid
should have done the trick.
Wolfgang, which version of Ubuntu are you using? At a fresh install of Ubuntu 12.04 LTS (without the transitional dhcp3-server package installed) the directory /var/run/ dhcp3-server/ does not exist.
For you, instead of patching the isc-dhcp-server script, dhcp3-server/ dhcpd.pid /var/run/dhcpd.pid
ln -s /var/run/
should have done the trick.