Remove configure switch --disable-strong-random

This removes a portion of infrastructure introduced by fe0a0b5 to allow
compilation of Postgres in environments where no strong random source is
available, meaning that there is no linking to OpenSSL and no
/dev/urandom (Windows having its own CryptoAPI).  No systems shipped
this century lack /dev/urandom, and the buildfarm is actually not
testing this switch at all, so just remove it.  This simplifies
particularly some backend code which included a fallback implementation
using shared memory, and removes a set of alternate regression output
files from pgcrypto.

Author: Michael Paquier
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/20181230063219.GG608@paquier.xyz

src/include/port.h

@@ -498,9 +498,12 @@ extern char *inet_net_ntop(int af, const void *src, int bits,
 			  char *dst, size_t size);
 
 /* port/pg_strong_random.c */
-#ifdef HAVE_STRONG_RANDOM
 extern bool pg_strong_random(void *buf, size_t len);
-#endif
+/*
+ * pg_backend_random used to be a wrapper for pg_strong_random before
+ * Postgres 12 for the backend code.
+ */
+#define pg_backend_random pg_strong_random
 
 /* port/pgcheckdir.c */
 extern int	pg_check_dir(const char *dir);