capability: add new ambient_capabilities_supported() helper

This new function reports whether ambient caps are available, and should
be quick because the result is cached.

src/test/test-capability.c

@@ -205,6 +205,8 @@ int main(int argc, char *argv[]) {
         log_parse_environment();
         log_open();
 
+        log_info("have ambient caps: %s", yes_no(ambient_capabilities_supported()));
+
         if (getuid() != 0)
                 return EXIT_TEST_SKIP;