capability-util: generalize helper to acquire local caps

This generalizes and modernizes the code to acquire set of local caps,
based on the code for this in the condition logic. Uses PidRef, and
acquires the full quintuplet of caps.

This can be considered preparation to one day maybe build without
libcap.

src/basic/capability-util.h

@@ -8,6 +8,7 @@
 
 #include "macro.h"
 #include "missing_capability.h"
+#include "pidref.h"
 
 /* Special marker used when storing a capabilities mask as "unset" */
 #define CAP_MASK_UNSET UINT64_MAX
@@ -84,3 +85,5 @@ bool capability_quintet_mangle(CapabilityQuintet *q);
 int capability_quintet_enforce(const CapabilityQuintet *q);
 
 int capability_get_ambient(uint64_t *ret);
+
+int pidref_get_capability(const PidRef *pidref, CapabilityQuintet *ret);