Added support for handling corrupted blocks

This provides a limited form of wear leveling. While wear is
not actually balanced across blocks, the filesystem can recover
from corrupted blocks and extend the lifetime of a device nearly
as much as dynamic wear leveling.

For use-cases where wear is important, it would be better to use
a full form of dynamic wear-leveling at the block level. (or
consider a logging filesystem).

Corrupted block handling was simply added on top of the existing
logic in place for the filesystem, so it's a bit more noodly than
it may have to be, but it gets the work done.

tests/test_format.sh

@@ -10,8 +10,8 @@ tests/test.py << TEST
 TEST
 
 echo "--- Invalid superblocks ---"
-ln -f -s /dev/null blocks/0
-ln -f -s /dev/null blocks/1
+ln -f -s /dev/zero blocks/0
+ln -f -s /dev/zero blocks/1
 tests/test.py << TEST
     lfs_format(&lfs, &cfg) => LFS_ERR_CORRUPT;
 TEST