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/test/test-capability.c

@@ -305,6 +305,18 @@ static void test_capability_get_ambient(void) {
         }
 }
 
+static void test_pidref_get_capability(void) {
+        CapabilityQuintet q = CAPABILITY_QUINTET_NULL;
+
+        assert_se(pidref_get_capability(&PIDREF_MAKE_FROM_PID(getpid_cached()), &q) >= 0);
+
+        assert_se(q.effective != CAP_MASK_UNSET);
+        assert_se(q.inheritable != CAP_MASK_UNSET);
+        assert_se(q.permitted != CAP_MASK_UNSET);
+        assert_se(q.effective != CAP_MASK_UNSET);
+        assert_se(q.ambient != CAP_MASK_UNSET);
+}
+
 int main(int argc, char *argv[]) {
         bool run_ambient;
 
@@ -336,5 +348,7 @@ int main(int argc, char *argv[]) {
 
         test_capability_get_ambient();
 
+        test_pidref_get_capability();
+
         return 0;
 }