Command injection via FoomaticRipCommandLine

[zdohnal]

Continuation of https://github.com/OpenPrinting/cups-filters/security/advisories/GHSA-p9rh-jxmq-gq47 , because the security advisory does not allow adding more comments after it is published.

The current plan:

• use hashed PPD keyword values to check whether the input is within expectation,
• provide root-only accessed config file, where user once he authenticates as root can add new hashed inputs, if he thinks the input is safe.