Switch nonce generation to purely random
Switch nonce generation to purely random

The old implementation of counting up from a random starting
point had the problem that is allowed an attacker to find out
the write order of the blocks.

davesteele/gocryptfs,rfjakob/gocryptfs,davesteele/gocryptfs,rfjakob/gocryptfs,davesteele/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs