Message queue name calculation

Message queue name calculation

- Use absolute path for directory name instead of base name.  This avoids issues
  with multiple directories with the same name, with trailing slashes on the
  directory name, and with symbolic links.
- Substitute slashes in the directory path with character 0xff to form a valid
  message queue name and avoid possible conflicts with real pathnames.
- Truncate the message queue name if it comes out to more than NAME_MAX bytes.