apply: Delay with login sessions and wall by default

[cgwalters]

At least the fedora-bootc base image enables bootc-fetch-apply-updates.service by default, which can be surprising to some, even though it's documented.

One thing that would likely help is to do the "detect login session, delay and use wall/broadcast" to notify; we can take relevant code from https://github.com/coreos/zincati/

This is also tangentially related to #1047