Stop using varargs in libkeyutils.

It's error-prone, and our specific usage of it here upsets ubsan.

Bug: https://issuetracker.google.com/158428513
Test: treehugger
Change-Id: I3a6b68865e6b4c37ac005f5f24c3d6e1de7c5bac
Merged-In: I3a6b68865e6b4c37ac005f5f24c3d6e1de7c5bac