What release of Slackware are you running?
The "eth_up" function is no longer used in Slackware 10.2 and onwards. My guess is that you installed some other (non-Slackware) software packages (like, I remember an old version of Dropline Gnome did this) that incorrectly patches the rc.inet1 file.
The rc.inet1 file initializes the network, and it can do so too for your wireless card if you want. But if you work with iwconfig and dhclient and find that sufficient, then you don't need rc.inet1 I guess.
And no, ldconfig does not need
to run on every boot. If it takes to long to start, comment it out of the boot script. Make sure you run ldconfig by hand then, each time you install new software.