Verify identity of process attaching to cdev

[droe]

The kext should verify the identity of the userspace process attaching to /dev/xnumon based on its code signature and refuse attaching if the code is unsigned or signed by the wrong team.

[droe]

This seems not to be easily possible with the current cdev interface and using supported KPIs only, because the KPIs in bsd/sys/codesign.h are private.

[droe]

Reference: https://forums.developer.apple.com/thread/108803