moss: Show a reboot message when a reboot is necessary.

[CookieSource]

When a package like the kernel is updated by Moss or another package that explicitly requires a reboot. (currently I can only think of the kernel)

Moss should output that a reboot is required and preferably which packages won't be updated until you reboot so it's clear to the user.

[ermo]

See also:

https://aerynos.dev/aerynos/faq/#when-do-i-need-to-reboot-after-updates

Phase 1 should probably be about showing users a contextual prompt of what they need to do.

In the future, once we enable on-disk format support changes, we might even be able to list this as a specific property per .stone, such that moss can aggregate the needs for either a relog XOR a reboot and initially warn about it, and then later enforce it (user-configurable re. enforcing).