t: Added lookahead to lfsr_traversal_t, adopted in lfs_alloc

This sort of turned into a complete refactor of lfs_alloc in order to
move/reuse the lookahead buffer filling logic into lfsr_fs_traverse.

lfs_alloc now calls lfsr_fs_traverse to fill the lookahead buffer when
no more blocks are available, but also you can too with lfsr_traversal_t
+ LFS_T_LOOKAHEAD.

The one big caveat being if any mutation happens to the filesystem, any
incomplete lookahead needs to be tossed out. To help with this,
lfsr_traversal_read now returns LFS_ERR_BUSY (-16) instead of
LFS_ERR_NOENT (-2) if the filesystem has been modified since the
traversal was opened.

Note that by default lfsr_traversal_t will still try to keep traversing
blocks, but can be told to terminate immediately with LFS_T_EXCL.
Continuing the traversal is probably desired for checking checksums,
debugging, etc, as otherwise you could end up looping over only the
first couple blocks in a write-heavy system, but if you are trying to
populate the lookahead buffer you probably want to just abort and start
over.

I considered adding a flags field to lfs_tinfo for this, but decided
against it since it would be the only place in the current API where we
don't use error codes to convey behavior-changing information. Though
this may be worth reconsidering at some point...

---

In reworking lfs_alloc, a lot of the internal logic was broken up into
specific functions:

- lfs_alloc_ckpoint - checkpoint the allocator
- lfs_alloc_discard - discard any lookahead
- lfs_alloc_shift - discard/shift lookahead if progress can be made
- lfs_alloc_markinuse - mark a block as in-use
- lfs_alloc_markfree - mark any remaining blocks as free
- lfs_alloc_findnext - find the next free block in lookahead

If anything this probably makes lfs_alloc more readable, though the
original motivation was to allow lfsr_traversal_t to only shift/zero the
lookahead buffer if there's a chance we can make progress.

This was based on upstream work by opilat and myself.

Code changes:

           code          stack
  before: 34226           2560
  after:  34474 (+0.7%)   2552 (-0.3%)

scripts/dbgerr.py

@@ -6,6 +6,7 @@ ERRS = [
     ('UNKNOWN',         -1,     "Unknown error"                     ),
     ('INVAL',           -22,    "Invalid parameter"                 ),
     ('NOTSUP',          -95,    "Operation not supported"           ),
+    ('BUSY',            -16,    "Device or resource busy"           ),
     ('IO',              -5,     "Error during device operation"     ),
     ('CORRUPT',         -84,    "Corrupted"                         ),
     ('NOENT',           -2,     "No directory entry"                ),