psa: Add initializers for key policies

Add new initializers for key policies and use them in our docs, example
programs, tests, and library code. Prefer using the macro initializers
due to their straightforwardness.
diff --git a/include/psa/crypto_struct.h b/include/psa/crypto_struct.h
index 44a1a60..320466f 100644
--- a/include/psa/crypto_struct.h
+++ b/include/psa/crypto_struct.h
@@ -208,4 +208,11 @@
     psa_algorithm_t alg;
 };
 
+#define PSA_KEY_POLICY_INIT {0, 0}
+static inline struct psa_key_policy_s psa_key_policy_init( void )
+{
+    const struct psa_key_policy_s v = PSA_KEY_POLICY_INIT;
+    return( v );
+}
+
 #endif /* PSA_CRYPTO_STRUCT_H */