t: Replaced LFS_T_EXCL with LFS_I_DIRTY flag in lfsr_tinfo

This just forwards the internal LFS_I_DIRTY flag to the user via the
lfsr_tinfo flags field.

Benefits of this approach:

- Gives the user more flexibility on what to do if the filesystem is
  modified, maybe you want to keep traversing depending on some other
  logic.

- Can eventually add other flags to tinfo.flags, such as
  LFS_I_COMPACTED, LFS_I_REPAIRED, LFS_I_INCONSISTENT, etc.

- Avoids confusion around the very different behaviors of LFS_O_EXCL and
  LFS_T_EXCL.

  I tried to come up with a better name (maybe LFS_T_WATCH?) but it was
  a bit of a struggle... Switching to a flags approach sidesteps the
  issue.

- Can drop the LFS_ERR_BUSY error code for now.

Code changes were fairly insignificant:

           code          stack
  before: 35244           2680
  after:  35224 (-0.1%)   2680 (+0.0%)

The only concern is that the tests highlighted it's possible for our
flag scheme to miss mutation if it happens after/during the last set of
blocks... Not sure how to handle this yet...

scripts/dbgerr.py

@@ -6,7 +6,6 @@ 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"                ),