Readded lfsr_fs_mkconsistent

See comments/previous commits. lfsr_fs_mkconsistent allows running
internal consistency operations without any other filesystem changes.

Implementation-wize, this just calls lfsr_fs_preparemutation which we
already need to, uh, prepare for mutation. Though it may do some
additional work in the future, such as setting compat flags, version
numbers, etc.

Added mkconsistent permutations to what seems like the relevant tests:

- test_forphans - easy for lfsr_fs_mkconsistent to accidentally delete
  orphans/zombies.

- test_powerloss - heavy fuzz tests over powerloss-related consistency
  operations, though this does multiply every permutation by ~2x...

Code cost minimal. I guess this is what it costs to make an internal
function non-static:

           code          stack
  before: 33634           2592
  after:  33642 (+0.0%)   2592 (+0.0%)

lfs.h

@@ -959,6 +959,18 @@ lfs_ssize_t lfsr_fs_size(lfs_t *lfs);
 // Returns a negative error code on failure.
 //int lfs_fs_traverse(lfs_t *lfs, int (*cb)(void*, lfs_block_t), void *data);
 
+#ifndef LFS_READONLY
+// Attempt to make the filesystem consistent and ready for writing
+//
+// Calling this function is not required, consistency will be implicitly
+// enforced on the first operation that writes to the filesystem, but this
+// function allows the work to be performed earlier and without other
+// filesystem changes.
+//
+// Returns a negative error code on failure.
+int lfsr_fs_mkconsistent(lfs_t *lfs);
+#endif
+
 #ifndef LFS_READONLY
 #ifdef LFS_MIGRATE
 // Attempts to migrate a previous version of littlefs