Add a new function nm_utils_random_bytes().

This function now preferably uses getrandom() syscall if it is
available.

As fallback, it always tries to fill the buffer from /dev/urandom.
If it cannot, as last fallback it uses GRand, which cannot fail.
Hence, the function always sets some (pseudo) random bytes.

It also returns FALSE if the obtained bytes are possibly not good
randomness.