Use arc4random_buf() instead of RAND_bytes() or RAND_pseudo_bytes().

arc4random provides high quality pseudo-random numbers, hence there is no
need to differentiate between "strong" and "pseudo". Furthermore, the
arc4random_buf() function is guaranteed to succeed, which avoids the need
to check for and handle failure, simplifying the code.

It is worth noting that a number of the replaced RAND_bytes() and
RAND_pseudo_bytes() calls were missing return value checks and these
functions can fail for a number of reasons (at least in OpenSSL -
thankfully they were converted to wrappers around arc4random_buf() some
time ago in LibreSSL).

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