Ressurect the nodes table pxe_boot_path column to allow specifying
an alternate "filename" for the dhcpd.conf file. "Inner plabs" are likely to need this (to boot pxelinux for emulating the plab boot CD). dhcpd_makeconf picks up the DB field and sets the filename field accordingly. nfree will invoke dhcpd_makeconf if any node being freed has a non-NULL pxe_boot_path value, in order to reset it.