Provide Assert() for frontend code.

Per discussion on-hackers. psql is converted to use the new code.

Follows a suggestion from Heikki Linnakangas.

src/include/postgres_fe.h

@@ -24,4 +24,16 @@
 
 #include "c.h"
 
+/*
+ * Assert() can be used in both frontend and backend code. In frontend code it
+ * just calls the standard assert, if it's available. If use of assertions is
+ * not configured, it does nothing.
+ */
+#ifdef USE_ASSERT_CHECKING
+#include <assert.h>
+#define Assert(p) assert(p)
+#else
+#define Assert(p)
+#endif
+
 #endif   /* POSTGRES_FE_H */