core: drop suid/sgid bit of files/dirs when doing recursive chown

This adds some extra paranoia: when we recursively chown a directory for
use with DynamicUser=1 services we'll now drop suid/sgid from all files
we chown().

Of course, such files should not exist in the first place, and noone
should get access to those dirs who isn't root anyway, but let's better
be safe than sorry, and drop everything we come across.

src/test/test-chown-rec.c

@@ -106,7 +106,7 @@ static void test_chown_recursive(void) {
         assert_se(st.st_gid == gid);
         assert_se(has_xattr(p));
 
-        assert_se(path_chown_recursive(t, 1, 2) >= 0);
+        assert_se(path_chown_recursive(t, 1, 2, 07777) >= 0);
 
         p = strjoina(t, "/dir");
         assert_se(lstat(p, &st) >= 0);