Reparse point updates

- Rename 'struct reparse_data' => 'struct link_reparse_point'
- Rename parse_reparse_data() => parse_link_reparse_data()
- Rename make_reparse_data() => make_link_reparse_point()
- Rename rp_unknown_1 => unknown_0x54 and move out of reparse point union
- Rename rp_unknown_2 => rp_reserved and store/restore
- Rename not_rpfixed => rp_flags and use 0x0001 as rpfix flag
- Use 'struct reparse_buffer_disk' in more places and not raw byte array
- Add new helper function: complete_reparse_point()
- Eliminate need for various assertions in reparse.c
- Improve handling of NT namespace paths in wim_inode_readlink()
- More closely follow POSIX readlink semantics in wim_inode_readlink()
- Set reparse tag and file attributes in wim_inode_readlink()
- Handle reparse point fixup in wim_inode_readlink() and support fixing
  up link targets in mounted WIM image
- Handle slashes between root and root-relative portion consistently
  between UNIX and Windows, and allow lossless handling of trailing
  slashes
- Use 0 as RP_NOT_FIXED in win32_apply.c
- Improve comments