Added on-mount traversal flags: LFS_M_MKCONSISTENT/CKMETA/CKDATA/etc

These tell littlefs to do the relevant gc work during mount, which may
be more convenient than calling lfsr_mount and then lfsr_fs_gc.

It also implicitly tears down the filesystem on error, which you can
imagine would be quite useful for LFS_M_CKMETA/LFS_M_CKDATA.

Some flags are more useful here than other (is LFS_M_LOOKAHEAD/COMPACT
really useful?), but since we just pass these directly to our traversal
APIs, we might as well support all of them for consistency.

Also note that since these only change mount's behavior, and have no
effect on the rest of the filesystem, these LFS_M_* flags don't have
related LFS_I_* flags and are not returned by lfsr_fs_stat.

---

This added quite a chunk of code, considering that this is entirely for
convenience:

           code          stack
  before: 35932           2680
  after:  36280 (+1.0%)   2680 (+0.0%)

But I think this is mostly because our low-level traversal state is
relatively costly to manage. It may be possible to deduplicate this a
bit better...

lfs.h

@@ -117,14 +117,21 @@ enum lfs_error {
 enum lfs_mount_flags {
     LFS_M_RDWR          = 0x0000, // Mount the filesystem as read and write
     LFS_M_RDONLY        = 0x0001, // Mount the filesystem as read only
-    LFS_M_CKPROGS       = 0x0010, // Check progs by reading back progged data
+    LFS_M_CKPROGS       = 0x0008, // Check progs by reading back progged data
+
+    LFS_M_MTREEONLY     = 0x0010, // Only traverse the mtree
+    LFS_M_MKCONSISTENT  = 0x0020, // Make the filesystem consistent
+    LFS_M_LOOKAHEAD     = 0x0040, // Populate lookahead buffer
+    LFS_M_COMPACT       = 0x0080, // Compact metadata logs
+    LFS_M_CKMETA        = 0x0100, // Check metadata checksums
+    LFS_M_CKDATA        = 0x0200, // Check metadata + data checksums
 };
 
 // Filesystem info flags
 enum lfs_fsinfo_flags {
     // mount flags
     LFS_I_RDONLY        = 0x0001, // Filesystem mounted read only
-    LFS_I_CKPROGS       = 0x0010, // Check progs by reading back progged data
+    LFS_I_CKPROGS       = 0x0008, // Check progs by reading back progged data
 
     // state flags
     LFS_I_INCONSISTENT  = 0x0100, // Filesystem needs mkconsistent to write