Replace PostmasterRandom() with a stronger source, second attempt.

This adds a new routine, pg_strong_random() for generating random bytes,
for use in both frontend and backend. At the moment, it's only used in
the backend, but the upcoming SCRAM authentication patches need strong
random numbers in libpq as well.

pg_strong_random() is based on, and replaces, the existing implementation
in pgcrypto. It can acquire strong random numbers from a number of sources,
depending on what's available:

- OpenSSL RAND_bytes(), if built with OpenSSL
- On Windows, the native cryptographic functions are used
- /dev/urandom

Unlike the current pgcrypto function, the source is chosen by configure.
That makes it easier to test different implementations, and ensures that
we don't accidentally fall back to a less secure implementation, if the
primary source fails. All of those methods are quite reliable, it would be
pretty surprising for them to fail, so we'd rather find out by failing
hard.

If no strong random source is available, we fall back to using erand48(),
seeded from current timestamp, like PostmasterRandom() was. That isn't
cryptographically secure, but allows us to still work on platforms that
don't have any of the above stronger sources. Because it's not very secure,
the built-in implementation is only used if explicitly requested with
--disable-strong-random.

This replaces the more complicated Fortuna algorithm we used to have in
pgcrypto, which is unfortunate, but all modern platforms have /dev/urandom,
so it doesn't seem worth the maintenance effort to keep that. pgcrypto
functions that require strong random numbers will be disabled with
--disable-strong-random.

Original patch by Magnus Hagander, tons of further work by Michael Paquier
and me.

Discussion: https://www.postgresql.org/message-id/CAB7nPqRy3krN8quR9XujMVVHYtXJ0_60nqgVc6oUk8ygyVkZsA@mail.gmail.com
Discussion: https://www.postgresql.org/message-id/CAB7nPqRWkNYRRPJA7-cF+LfroYV10pvjdz6GNvxk-Eee9FypKA@mail.gmail.com

contrib/pgcrypto/px.c

@@ -51,7 +51,6 @@ static const struct error_desc px_err_list[] = {
 	{PXE_CIPHER_INIT, "Cipher cannot be initialized ?"},
 	{PXE_HASH_UNUSABLE_FOR_HMAC, "This hash algorithm is unusable for HMAC"},
 	{PXE_DEV_READ_ERROR, "Error reading from random device"},
-	{PXE_OSSL_RAND_ERROR, "OpenSSL PRNG error"},
 	{PXE_BUG, "pgcrypto bug"},
 	{PXE_ARGUMENT_ERROR, "Illegal argument to function"},
 	{PXE_UNKNOWN_SALT_ALGO, "Unknown salt algorithm"},
@@ -86,6 +85,39 @@ static const struct error_desc px_err_list[] = {
 	{0, NULL},
 };
 
+/*
+ * Call ereport(ERROR, ...), with an error code and message corresponding to
+ * the PXE_* error code given as argument.
+ *
+ * This is similar to px_strerror(err), but for some errors, we fill in the
+ * error code and detail fields more appropriately.
+ */
+void
+px_THROW_ERROR(int err)
+{
+	if (err == PXE_NO_RANDOM)
+	{
+#ifdef HAVE_STRONG_RANDOM
+		ereport(ERROR,
+				(errcode(ERRCODE_INTERNAL_ERROR),
+				 errmsg("could not generate a random number")));
+#else
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("pg_random_bytes() is not supported by this build"),
+				 errdetail("This functionality requires a source of strong random numbers"),
+				 errhint("You need to rebuild PostgreSQL using --enable-strong-random")));
+#endif
+	}
+	else
+	{
+		/* For other errors, use the message from the above list. */
+		ereport(ERROR,
+				(errcode(ERRCODE_EXTERNAL_ROUTINE_INVOCATION_EXCEPTION),
+				 errmsg("%s", px_strerror(err))));
+	}
+}
+
 const char *
 px_strerror(int err)
 {