Consider replacing math/rand with crypto/rand in snapd

[Prabhu Subramanian]

https://github.com/snapcore/snapd/blob/master/randutil/rand.go#L28

As per the official documentation, crypto/rand should be used for security-sensitive work

[Ian Johnson (anonymouse67)]

Hi, we do use crypto/rand for various purposes, but there are specific situations where we don't need cryptographic random numbers and psuedo random numbers are suitable in snapd.