Add support for getentropy() and getrandom()

[colmmacc]

Some platforms have a getentropy() call for retrieving randomly generated data, Linux has recently added getrandom(), an equivalent call. s2n should support these where available and avoid using /dev/urandom.

[masklinn]

Beware, as the name indicates getentropy(2) is a source of entropy not a direct source of randomness. getentropy(2) data should be fed into a PRNG, not used directly as randomness as the man page indicates:

getentropy() is not intended for regular code; please use the arc4random(3) family of functions instead.

[goatgoose]

We don't want to further complicate our random implementation, and plan to use the libcrypto random when possible:
#4348