Add warn_unused_result attributes to rng_(get|allocate)_bytes_not_zero

Also fixed Doxygen comments.

src/libstrongswan/crypto/rngs/rng.h

@@ -82,10 +82,11 @@ struct rng_t {
  * @param rng			rng_t object
  * @param len			number of bytes to get
  * @param buffer		pointer where the generated bytes will be written
- * @param all			TRUE if all bytes have to be non-zero
+ * @param all			TRUE if all bytes have to be non-zero, FALSE for first
  * @return				TRUE if bytes successfully written
  */
-bool rng_get_bytes_not_zero(rng_t *rng, size_t len, u_int8_t *buffer, bool all);
+bool rng_get_bytes_not_zero(rng_t *rng, size_t len, u_int8_t *buffer,
+							bool all) __attribute__((warn_unused_result));
 
 /**
  * Wrapper around rng_t.allocate_bytes() ensuring that either all bytes or at
@@ -93,12 +94,12 @@ bool rng_get_bytes_not_zero(rng_t *rng, size_t len, u_int8_t *buffer, bool all);
  *
  * @param rng			rng_t object
  * @param len			number of bytes to get
- * @param buffer		pointer where the generated bytes will be written
+ * @param chunk			chunk that stores the generated bytes (allocated)
- * @param all			TRUE if all bytes have to be non-zero
+ * @param all			TRUE if all bytes have to be non-zero, FALSE for first
  * @return				TRUE if bytes successfully written
  */
 bool rng_allocate_bytes_not_zero(rng_t *rng, size_t len, chunk_t *chunk,
-								 bool all);
+								 bool all) __attribute__((warn_unused_result));