Fix various issues in cupsd

Fix various issues in cupsd

Various issues were found by @SilverPlate3, recognized as CVE-2025-61915:

- out of bound write when handling IPv6 addresses,
- cupsd crash caused by null dereference when ErrorPolicy value is empty,

On the top of that, Mike Sweet noticed vulnerability via domain socket,
exploitable locally if attacker has access to domain socket and knows username
of user within a group which is present in CUPS system groups:

- rewrite of cupsd.conf via PeerCred authorization via domain socket

The last vulnerability is fixed by introducing PeerCred directive for cups-files.conf,
which controls whether PeerCred is enabled/disabled for user in CUPS system groups.

Fixes CVE-2025-61915