Added LFS_O_CKMETA/CKDATA flags

These flags just call lfsr_file_ckmeta/ckdata under the hood, but make
it very easy to check metadata/data when opening a file. As an extra
plus they implicitly close the file on failure, so might make cleanup
easier.

Of course, everything has a cost:

           code          stack
  before: 36368           2664
  after:  36424 (+0.2%)   2664 (+0.0%)

These also ruin my previous "you don't pay for what you don't call"
assertion, since runtime flags unfortunately always pull in code.

We should add a compile-time switch for these evntually.

lfs.h

@@ -136,6 +136,8 @@ enum lfs_type {
 #define LFS_O_FLUSH     0x00000040  // Flush data on every write
 #define LFS_O_SYNC      0x00000080  // Sync metadata on every write
 #define LFS_O_DESYNC    0x00000100  // Do not sync or recieve file updates
+#define LFS_O_CKMETA    0x00001000  // Check metadata checksums
+#define LFS_O_CKDATA    0x00002000  // Check metadata + data checksums
 
 // internally used flags
 #define LFS_F_TYPE      0xff000000  // The file's type