main/ipset: add netns support

Background: openrc got patched to support running services in netns namespaces !51670 (merged)

This MR adds netns support for the ipset initd script. It also makes the DIR variable configurable so it is possible to use a different ipset config in netns namespaces.

Example /etc/conf.d/ipset.tenant1:

# enable netns support
netns=tenant1

# use custom ipset config dir
DIR=/etc/ipset.tenant1.d