Listing processes that use old files

[Kete Foy]

Enhancement request for a notice of processes running old files after 
upgrading a package.
In this case, the upgraded package would be a dependency of another process.
This is crucial for running patched software without security 
vulnerabilities.
A guix command for listing the processes would be nice, but we could 
lsof in the meantime.
The package manager would not take any further action.
We discussed this in the IRC channel.