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.h

@@ -71,7 +71,6 @@ void		px_free(void *p);
 #define PXE_CIPHER_INIT				-8
 #define PXE_HASH_UNUSABLE_FOR_HMAC	-9
 #define PXE_DEV_READ_ERROR			-10
-#define PXE_OSSL_RAND_ERROR			-11
 #define PXE_BUG						-12
 #define PXE_ARGUMENT_ERROR			-13
 #define PXE_UNKNOWN_SALT_ALGO		-14
@@ -189,11 +188,7 @@ int			px_find_hmac(const char *name, PX_HMAC **res);
 int			px_find_cipher(const char *name, PX_Cipher **res);
 int			px_find_combo(const char *name, PX_Combo **res);
 
-int			px_get_random_bytes(uint8 *dst, unsigned count);
-int			px_add_entropy(const uint8 *data, unsigned count);
-
-unsigned	px_acquire_system_randomness(uint8 *dst);
-
+void		px_THROW_ERROR(int err) pg_attribute_noreturn();
 const char *px_strerror(int err);
 
 const char *px_resolve_alias(const PX_Alias *aliases, const char *name);