post-install: enable systemd-userdbd.socket by default
This used to be statically-enabled, but is made non-static (installable) in https://github.com/systemd/systemd/commit/ead7af30939e3f1903455c7e06bc7cfeef54d4a2. As stated in commit message, it is generally useful, and I see no reason not to ship it by default. People who don't want it can disable it on their own.