Replace isc-dhcp-client with dhcpcd-base for sid, forky and kali

isc-dhcp-client is being phased out in Debian, and it was just removed from forky/testing due to https://bugs.debian.org/1106121

Replacing it with dhcpcd-base (as it Provides: dhcp-client, just like isc-dhcp-client) seems to do the trick.

Even though the isc-dhcp-client is still in sid/unstable at the moment, it's planned to be removed, so let's use dhcpcd-base for those suites as well.

This is temporary: a better solution will be to land the fix in lxc-templates (cf. https://bugs.debian.org/1125011), and in the long run, debci will switch away from using old lxc-templates, in favor of e.g distrobuilder, or something else.