Reworked file flushing to allow cached data to remain in buffer

This is intended to enable lfsr_file_read to use the buffer as well.

This adds LFS_F_UNFLUSHED and internal lfsr_file_flush to manage buffer
flushing. This also results in a nice reorganization of lfsr_file_sync.

Of course, small file caching proves to be a big pain again, with
several subtle corner cases in truncate/fruncate that need to set the
LFS_F_UNFLUSHED flag so we fix small files in lfsr_file_sync.

lfs.h

@@ -165,8 +165,9 @@ enum lfs_open_flags {
 #endif
 
     // internally used flags
-    LFS_F_UNSYNCED  = 0x010000, // File's metadata does not match storage
-    LFS_F_ERRORED   = 0x020000, // An error occurred during write
+    LFS_F_UNFLUSHED = 0x010000, // File's data does not match storage
+    LFS_F_UNSYNCED  = 0x020000, // File's metadata does not match storage
+    LFS_F_ERRORED   = 0x040000, // An error occurred during write
 };
 
 // File seek flags