Adds random nonce (r) option to `hf mf sim`.

This makes the PM3 generate pseudo-random nonces rather than sequential
nonces, to make it act a bit more like a "real" MFC card.  A reader would
otherwise be able to detect the PM3 probing based on the predictable nonces
and throw different authentication challenges (or refuse to authenticate at
all).

The code includes an implementation of a rand-like function (prand), similar
to the one from libc, which is seeded automatically based on the time it
takes between the PM3 starting up and the first call to the RNG.

This isn't cryptographically random, but should be "good enough" to be able
to evade basic detection.