Replace random(), pg_erand48(), etc with a better PRNG API and algorithm.

Standardize on xoroshiro128** as our basic PRNG algorithm, eliminating
a bunch of platform dependencies as well as fundamentally-obsolete PRNG
code.  In addition, this API replacement will ease replacing the
algorithm again in future, should that become necessary.

xoroshiro128** is a few percent slower than the drand48 family,
but it can produce full-width 64-bit random values not only 48-bit,
and it should be much more trustworthy.  It's likely to be noticeably
faster than the platform's random(), depending on which platform you
are thinking about; and we can have non-global state vectors easily,
unlike with random().  It is not cryptographically strong, but neither
are the functions it replaces.

Fabien Coelho, reviewed by Dean Rasheed, Aleksander Alekseev, and myself

Discussion: https://postgr.es/m/alpine.DEB.2.22.394.2105241211230.165418@pseudo

src/backend/postmaster/postmaster.c

@@ -98,6 +98,7 @@
 #include "catalog/pg_control.h"
 #include "common/file_perm.h"
 #include "common/ip.h"
+#include "common/pg_prng.h"
 #include "common/string.h"
 #include "lib/ilist.h"
 #include "libpq/auth.h"
@@ -2699,19 +2700,19 @@ ClosePostmasterPorts(bool am_syslogger)
 void
 InitProcessGlobals(void)
 {
-	unsigned int rseed;
-
 	MyProcPid = getpid();
 	MyStartTimestamp = GetCurrentTimestamp();
 	MyStartTime = timestamptz_to_time_t(MyStartTimestamp);
 
 	/*
-	 * Set a different seed for random() in every process.  We want something
+	 * Set a different global seed in every process.  We want something
 	 * unpredictable, so if possible, use high-quality random bits for the
 	 * seed.  Otherwise, fall back to a seed based on timestamp and PID.
 	 */
-	if (!pg_strong_random(&rseed, sizeof(rseed)))
+	if (unlikely(!pg_prng_strong_seed(&pg_global_prng_state)))
 	{
+		uint64		rseed;
+
 		/*
 		 * Since PIDs and timestamps tend to change more frequently in their
 		 * least significant bits, shift the timestamp left to allow a larger
@@ -2722,8 +2723,17 @@ InitProcessGlobals(void)
 		rseed = ((uint64) MyProcPid) ^
 			((uint64) MyStartTimestamp << 12) ^
 			((uint64) MyStartTimestamp >> 20);
+
+		pg_prng_seed(&pg_global_prng_state, rseed);
 	}
-	srandom(rseed);
+
+	/*
+	 * Also make sure that we've set a good seed for random(3).  Use of that
+	 * is deprecated in core Postgres, but extensions might use it.
+	 */
+#ifndef WIN32
+	srandom(pg_prng_uint32(&pg_global_prng_state));
+#endif
 }