Use a separate random seed for SQL random()/setseed() functions.

Use a separate random seed for SQL random()/setseed() functions.

Previously, the SQL random() function depended on libc's random(3),
and setseed() invoked srandom(3).  This results in interference between
these functions and backend-internal uses of random(3).  We'd never paid
too much mind to that, but in the wake of commit 88bdbd3f7 which added
log_statement_sample_rate, the interference arguably has a security
consequence: if log_statement_sample_rate is active then an unprivileged
user could probably control which if any of his SQL commands get logged,
by issuing setseed() at the right times.  That seems bad.

To fix this reliably, we need random() and setseed() to use their own
private random state variable.  Standard random(3) isn't amenable to such
usage, so let's switch to pg_erand48().  It's hard to say whether that's
more or less "random" than any particular platform's version of random(3),
but it does have a wider seed value and a longer period than are required
by POSIX, so we can hope that this isn't a big downgrade.  Also, we should
now have uniform behavior of random() across platforms, which is worth
something.

While at it, upgrade the per-process seed initialization method to use
pg_strong_random() if available, greatly reducing the predictability
of the initial seed value.  (I'll separately do something similar for
the internal uses of random().)

In addition to forestalling the possible security problem, this has a
benefit in the other direction, which is that we can now document
setseed() as guaranteeing a reproducible sequence of random() values.
Previously, because of the possibility of internal calls of random(3),
we could not promise any such thing.

Discussion: https://postgr.es/m/3859.1545849900@sss.pgh.pa.us