reverse: rename readFile to clearer readBackingFile
reverse: rename readFile to clearer readBackingFile

Also refactor the header generation for nicer code.

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