tree-wide: symlink well-known Varlink service entry point sockets into /run/varlink...

This is generally useful, but is particularly useful in context of
https://github.com/mvo5/varlink-proxy-rs which can expose a set of local
Varlink services via a HTTP bridge. The idea is that the sockets linked
into /run/varlink/registry/ are candidates for being exposed like that.

/cc @mvo5