label: rework label_fix() implementations (#8583)

This reworks the SELinux and SMACK label fixing calls in a number of
ways:

1. The two separate boolean arguments of these functions are converted
   into a flags type LabelFixFlags.

2. The operations are now implemented based on O_PATH. This should
   resolve TTOCTTOU races between determining the label for the file
   system object and applying it, as it it allows to pin the object
   while we are operating on it.

3. When changing a label fails we'll query the label previously set, and
   if matches what we want to set anyway we'll suppress the error.

Also, all calls to label_fix() are now (void)ified, when we ignore the
return values.

Fixes: #8566

src/tmpfiles/tmpfiles.c

@@ -850,7 +850,7 @@ static int fd_set_perms(Item *i, int fd, const struct stat *st) {
         }
 
 shortcut:
-        return label_fix(path, false, false);
+        return label_fix(path, 0);
 }
 
 static int path_set_perms(Item *i, const char *path) {