mount_image.c: Cleanup

mount_image.c: Cleanup

- Improve some comments.
- Don't use the T() macro.  The mount code doesn't run on Windows.
- Make sure we don't accidentally return errno=0.
- Return WIMLIB_ERR_NO_FILENAME if read-write mounting WIM that has no
  backing file.
- inode_resolve_streams() already does nothing if inode->i_resolved
- create_dentry() is never called with NULL 'dentry_ret'
- attr_timeout=0 was in FUSE options two times!
- 'have_progfunc' argument to create_message_queue() was always true; get
  rid of it.
- Use geteuid() rather than getuid() when testing for root