reverse: Implement unique IV derived from the inode number

This implements per-file unique IVs for reverse mode and enables them by default, as the security improvement is significant. The SHA1-hashed inode number is used as the IV, the comments in 73a5accb8ea5d975238ff8f9c538e871c72714ea explain the details.
Even tough it looks quite big, a good part of the series is just adding comments to existing code and improving the test suite.
On the way, I found that FUSE's caching can lead to stale data being returned for reverse mode. 84dae17d4e01967bf7bb58c23b18c6c7b2f9a728 disables caching for reverse mode (details in the comments). Also, the normal mode got a "--nocache" option to enable proper testing of the encryption - decryption chain. When mounting a reverse-encrypted filesystem, this ensures you don't get stale data.