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.

src/nspawn/nspawn.c

@@ -51,7 +51,7 @@
 #include "blkid-util.h"
 #include "btrfs-util.h"
 #include "cap-list.h"
-#include "capability.h"
+#include "capability-util.h"
 #include "cgroup-util.h"
 #include "copy.h"
 #include "dev-setup.h"