Added LFS_T_RDONLY, LFS_T_RDWR, etc

These mimic the relevant LFS_O_* flags, and allow users to assert
whether or not a traversal will mutate the filesystem:

  LFS_T_MODE          0x00000001  The traversal's access mode
  LFS_T_RDWR          0x00000000  Open traversal as read and write
  LFS_T_RDONLY        0x00000001  Open traversal as read only

In theory, these could also change internal allocations, but littlefs
doesn't really work that way.

Note we _don't_ add related LFS_GC_RDONLY, LFS_GC_RDWR, etc flags. These
are sort of implied by the relevant LFS_M_* flags.

Adds a bit more code, probably because of the slightly more complicated
internal constants for the internal traversals. But I think the
self-documentingness is worth it:

           code          stack          ctx
  before: 37200           2288          636
  after:  37220 (+0.1%)   2288 (+0.0%)  636 (+0.0%)

lfs.h

@@ -270,7 +270,10 @@ enum lfs_btype {
 };
 
 // Traversal flags
-#define LFS_T_MTREEONLY 0x00000004  // Only traverse the mtree
+#define LFS_T_MODE               1  // The traversal's access mode
+#define LFS_T_RDWR               0  // Open traversal as read and write
+#define LFS_T_RDONLY             1  // Open traversal as read only
+#define LFS_T_MTREEONLY 0x00000002  // Only traverse the mtree
 #define LFS_T_MKCONSISTENT \
                         0x00000100  // Make the filesystem consistent
 #define LFS_T_LOOKAHEAD 0x00000200  // Populate lookahead buffer