Fixed data corruption with multiple write handles

Multiple write handles in littlefs has always been a bit confusing
(hopefully improving in littlefs3), but I didn't realize it could lead
to corrupted data.

The problem, as noted by Ictogan1, is that syncs to related file handles
ignores the LFS_F_DIRTY flag. If you open a file twice, and write to one
handle, littlefs doesn't realize the other handle it out-of-date.

This may not seem like a problem, but then littlefs is happy to
reallocate those (still referenced) blocks, leading to data corruption:

  open(a, "quiche.txt")
  write(a)
  sync(a) // syncs a's contents
  open(b, "quiche.txt")
  truncate(b)
  sync(b) // syncs b's contents
  write(b) // may allocate from a
  rewind(a)
  read(a) // potentially corrupted

---

What we want is to set LFS_F_DIRTY in all other file handles during
lfs_file_sync, but doing so would force those file handles to sync
during close. That would be even more confusing (not to mention
backwards incompatible).

In theory setting both LFS_F_DIRTY + LFS_F_ERRED could work, but that
would prevent implicit syncs of writes that haven't actually errored:

  open(a, "quiche.txt")
  write(a)
  open(b, "quiche.txt")
  write(b)
  close(b) // syncs b's contents
  close(a) // should sync a's contents

So, instead, as a somewhat clunky workaround, a new flag: LFS_F_DUSTY,
which indicates a file does not match storage, but should not be synced
during close.

---

It's worth noting this is already fixed in littlefs3, which includes a
more rigorous, and hopefully easier to use sync model. But in the
meantime, this should at least prevent the loss of data.

Added test_alloc_multihandle and test_alloc_multihandle_reuse to prevent
a regression, test_alloc_multihandle_reuse does reproduce the bug.

Found and reproduced by Ictogan1

lfs.h

@@ -135,14 +135,15 @@ enum lfs_open_flags {
 
     // internally used flags
 #ifndef LFS_READONLY
-    LFS_F_DIRTY   = 0x010000, // File does not match storage
-    LFS_F_WRITING = 0x020000, // File has been written since last flush
+    LFS_F_DIRTY   = 0x00010000, // File does not match storage due to write
+    LFS_F_DUSTY   = 0x00020000, // File does not match storage due to desync
+    LFS_F_WRITING = 0x00040000, // File has been written since last flush
 #endif
-    LFS_F_READING = 0x040000, // File has been read since last flush
+    LFS_F_READING = 0x00080000, // File has been read since last flush
 #ifndef LFS_READONLY
-    LFS_F_ERRED   = 0x080000, // An error occurred during write
+    LFS_F_ERRED   = 0x00100000, // An error occurred during write
 #endif
-    LFS_F_INLINE  = 0x100000, // Currently inlined in directory entry
+    LFS_F_INLINE  = 0x01000000, // Currently inlined in directory entry
 };
 
 // File seek flags