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...

lfs.h

@@ -98,7 +98,6 @@ enum lfs_error {
     LFS_ERR_UNKNOWN     = -1,   // Unknown error
     LFS_ERR_INVAL       = -22,  // Invalid parameter
     LFS_ERR_NOTSUP      = -95,  // Operation not supported
-    LFS_ERR_BUSY        = -16,  // Device or resource busy
     LFS_ERR_IO          = -5,   // Error during device operation
     LFS_ERR_CORRUPT     = -84,  // Corrupted
     LFS_ERR_NOENT       = -2,   // No directory entry
@@ -169,8 +168,7 @@ enum lfs_btype {
 // Traversal flags
 enum lfs_traversal_flags {
     // traversal open flags
-    LFS_T_MTREEONLY     = 0x0008, // Only traverse the mtree
-    LFS_T_EXCL          = 0x0010, // Terminate if filesystem modified
+    LFS_T_MTREEONLY     = 0x0010, // Only traverse the mtree
     LFS_T_MKCONSISTENT  = 0x0020, // Make the filesystem consistent
     LFS_T_LOOKAHEAD     = 0x0040, // Populate lookahead buffer
     LFS_T_COMPACT       = 0x0080, // Compact metadata logs
@@ -179,9 +177,11 @@ enum lfs_traversal_flags {
 // TODO
 //    LFS_T_REPAIRMETA    = 0x0400, // Repair metadata blocks
 //    LFS_T_REPAIRDATA    = 0x0c00, // Repair metadata + data blocks
+};
 
-    // internally used flags
-    LFS_F_DIRTY         = 0x1000, // Filesystem has been modified
+enum lfs_tinfo_flags {
+    // traversal info flags
+    LFS_I_DIRTY         = 0x1000, // Filesystem has been modified
 };
 
 
@@ -385,6 +385,9 @@ struct lfs_fsinfo {
 
 // Traversal info structure
 struct lfs_tinfo {
+    // Traversal flags
+    uint16_t flags;
+
     // Type of the block
     uint8_t btype;