Replaced rattr.u.etc with rattr relevant types

This does a couple things:

- Makes attr-lists a bit more self-documenting.

- Adds a bit more type-safety. The LFSR_RATTR_* macros should be able to
  reject types that don't match the expected encoding.

- Makes it easier to adjust dsize estimates at one location.

  Specifically, this makes it harder to forget bptr's LFSR_BPTR_DSIZE.

---

Surprisingly this did have a small impact on code size. I'm not entirely
sure why, but considering how much of the codebase this touches I'm just
going to chalk this up to compiler noise:

           code          stack          ctx
  before: 35488           2440          636
  after:  35536 (+0.1%)   2440 (+0.0%)  636 (+0.0%)

lfsr_file_carve seems the hardest hit:

  function (0 added, 0 removed)      osize    nsize    dsize
  lfsr_file_open                        16       20       +4 (+25.0%)
  lfsr_file_carve                     1316     1356      +40 (+3.0%)
  lfsr_remove                          408      412       +4 (+1.0%)
  TOTAL                              35488    35536      +48 (+0.1%)

Author: geky