Use explicit_bzero

Use the explicit_bzero() function in places where it is important that
security information such as passwords is cleared from memory.  There
might be other places where it could be useful; this is just an
initial collection.

For platforms that don't have explicit_bzero(), provide various
fallback implementations.  (explicit_bzero() itself isn't standard,
but as Linux/glibc, FreeBSD, and OpenBSD have it, it's the most common
spelling, so it makes sense to make that the invocation point.)

Discussion: https://www.postgresql.org/message-id/flat/42d26bde-5d5b-c90d-87ae-6cab875f73be%402ndquadrant.com

src/port/explicit_bzero.c

@@ -0,0 +1,55 @@
+/*-------------------------------------------------------------------------
+ *
+ * explicit_bzero.c
+ *
+ * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/port/explicit_bzero.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "c.h"
+
+#if defined(HAVE_MEMSET_S)
+
+void
+explicit_bzero(void *buf, size_t len)
+{
+	(void) memset_s(buf, len, 0, len);
+}
+
+#elif defined(WIN32)
+
+void
+explicit_bzero(void *buf, size_t len)
+{
+	(void) SecureZeroMemory(buf, len);
+}
+
+#else
+
+/*
+ * Indirect call through a volatile pointer to hopefully avoid dead-store
+ * optimisation eliminating the call.  (Idea taken from OpenSSH.)  We can't
+ * assume bzero() is present either, so for simplicity we define our own.
+ */
+
+static void
+bzero2(void *buf, size_t len)
+{
+	memset(buf, 0, len);
+}
+
+static void (* volatile bzero_p)(void *, size_t) = bzero2;
+
+void
+explicit_bzero(void *buf, size_t len)
+{
+	bzero_p(buf, len);
+}
+
+#endif