Use rebuild when triggering safe mode from frontend

[agners]

Description

Currently, the frontend uses the service call homeassistant.restart to trigger restart (with safe mode as attribute). The hassio integration registers for the "stop_handler" (see homeassistant/components/hassio/init.py), which then calls /homeassistant/restart.

Goal: Home Assistant restart into safe mode triggered from UI should rebuild when Supervisor is pressent

Option 1: Make Supervisor detect if the safe mode marker file is present, and "convert" the restart to a rebuild. This seems awkward, and "bends" the wish of the Core/hassio integration.

Option 2: Make hassio integration be aware that restart into safe mode is requested (extend the stop_handler), and request a /homeassistant/rebuild in that case. With that, Supervisor is not aware at all.

Additional context

No response

[agners]

Let's go for Option 2 here, this seems much cleaner to me and limits the changes to Core.