This is needed to avoid a breaking change to the volatile keys in Incus
and potentially in other tools using the serialized JSON format.

Signed-off-by: Stéphane Graber <stgraber@stgraber.org>

This properly documents the working of AddSafe as well as fixes an issue
where a new entry that's of both UID and GID types could intersect with
more than one entry (if mixed with entries that aren't both UID and GID).

Simply keep track of whether the entry was already added and if it did,
process the intersection but don't add the entry again.

Signed-off-by: Stéphane Graber <stgraber@stgraber.org>

Signed-off-by: Stéphane Graber <stgraber@stgraber.org>