src/basic: rename audit.[ch] → audit-util.[ch] and capability.[ch] → capability-util...

src/basic: rename audit.[ch] → audit-util.[ch] and capability.[ch] → capability-util.[ch]

The files are named too generically, so that they might conflict with
the upstream project headers. Hence, let's add a "-util" suffix, to
clarify that this are just our utility headers and not any official
upstream headers.