This commit adds in RandReader, a cryptographically secure io.Reader
that will fail when the os has insufficient randomness. This is done
using the getrandom() syscall in non-blocking mode.
see: http://man7.org/linux/man-pages/man2/getrandom.2.html
Any kernel new enough to have filesystem encryption will also have this
syscall.
This RandReader is preferable to the one provided by the standard
library in crypto/rand. See the bugs:
https://github.com/golang/go/issues/11833
https://github.com/golang/go/issues/19274
This will be removed when go updates the crypto/rand implementation.