Allow all domains to connect to systemd-nsresourced over a unix socket

[zpytela]

Each subsystem that needs to define users and groups on the local system is supposed to implement user-group API [1] and offer its interfaces on a Varlink AF_UNIX/SOCK_STREAM file system socket bound into the /run/systemd/userdb/ directory.

[1] https://systemd.io/USER_GROUP_API/