I take it your current version of Slackware is 12.1 ? I think the network startup scripts have changed since Slackware 10 so the following may not work on older versions.
Not sure if this will work or even if it's the best way of doing things but dhcpcd man page reports an infinite timeout may be set by setting the timeout parameter, ( -t ), to 0.
I believe this may be achieved by adding a setting to /etc/rc.d/rc.inet1.conf. Add a line to the file:-
- where X is the interface number - i.e. set X to 0 for eth0, 1 for eth1 etc.
Looking at the /etc/rc.d/rc.inet1 script this should set the -t value of the dhcpcd call to 0.
It may work but as I said - this may not be the best way of getting the desired results.