When entropy gathering is implemented, in the case of the installer and
upgrader, the system probably won't have any entropy when it begins. By the
time the system is installed, there will probably be a bit of entropy from
the user using the system and general system usage, so mix in some of that.
In the case of init, after an installed system has run for a while, a lot of
entropy will have gotten collected, but init will have its arc4random seeded
with initial boot entry, so mix in some fresh entropy, so the random seed
written on shutdown remains as entropic as possible.
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