Synchronise the blacklisted packages across users.

[TheKingOfLight]

Problem:
In LMDE 6 with MintUpdate 7.0.7, blacklisting updates for a package currently only affects the current user.
Even if I lock the version of a package in MintUpdate, another user could still unintentionally update the package (either manually or via automatic updates), which could result in a broken system.

Solution:
Use a global blacklist to synchronise blacklisted packages across users. This will prevent the accidental installation of unwanted updates.

Ideally, the blacklist would use the "hold" feature in apt/dpkg to prevent accidental installation, even when using apt directly.

Additional context:
Synaptic's "lock" feature locks packages for all users.