Several fatal errors were just printed to stdout, which
meant they were invisible when running the test suite.
Fix this by introducing toggledlog.Fatal and convert as
follows:
Fatal errors -> toggledlog.Fatal
Warnings -> toggledlog.Warn
Password prompts -> fmt.Fprintf
Go GCM is faster than OpenSSL if the CPU has AES instructions
and you are running Go 1.6+.
The "-openssl" option now defaults to "auto".
"gocryptfs -debug -version" displays the result of the autodetection.
See https://github.com/rfjakob/gocryptfs/issues/23 for details and
benchmarks.