SecureRandom.random_bytes: assume zero entropy for seed value
If OpenSSL is available SecureRandom.random_bytes uses
OpenSSL::Random.random_bytes and the random generator is reseeded 
whenever the current pid changes (due to repeated values when a pid
is reused, cf. #4579).
Since this seeding is also called the first time the method is entered,
using OpenSSL::Random.seed is potentially dangerous. OpenSSL::Random.seed
is equal to using OpenSSL::Random.random_add where it is assumed that the
string passed to seed possesses full entropy. This is definitely not the
case for pid and time values. In fact, OpenSSL itself assumes an entropy
of 1.0 or even 0.0 when doing similar seeding in RAND_poll . However,
this seems to have no impact so far, since the OpenSSL random generator
gathers enough entropy on startup even if we seeded with what it would
consider enough bytes of entropy (32 by default). So even if our seed
string is already 32 bytes or larger, OpenSSL's RAND_poll still seems to
collect 32 bytes of entropy on initialization regardless of what has been
added/seeded so far, which is a good thing in this case. Still, this could
change over time if OpenSSL for example changes internal behaviour and
would decide that enough entropy had been provided while seeding.
Therefore I believe using OpenSSL::Random.random_add with an assumed
entropy of 0.0 might be a more defensive choice. The forking test from
#4579 still passes with the attached patch. What do you think?