rand: treat fake entropy the same regardless of endianness

When the random seed is purposely made predictable for testing purposes
by using the CURL_ENTROPY environment variable, process that data in an
endian agnostic way so the the initial random seed is the same
regardless of endianness.

- Change Curl_rand to write to a char array instead of int array.

- Add Curl_rand_hex to write random hex characters to a buffer.

Fixes #1315
Closes #1468

Co-authored-by: Daniel Stenberg
Reported-by: Michael Kaufmann

lib/rand.h

@@ -7,7 +7,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2016, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2017, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -23,7 +23,7 @@
  ***************************************************************************/
 
 /*
- * Curl_rand() stores 'num' number of random unsigned integers in the buffer
+ * Curl_rand() stores 'num' number of random unsigned characters in the buffer
  * 'rnd' points to.
  *
  * If libcurl is built without TLS support or with a TLS backend that lacks a
@@ -37,7 +37,11 @@
  * easy handle!
  *
  */
-CURLcode Curl_rand(struct Curl_easy *data, unsigned int *rnd,
-                   unsigned int num);
+CURLcode Curl_rand(struct Curl_easy *data, unsigned char *rnd, size_t num);
+
+/* Same as above but outputs only random lowercase hex characters.
+   Does NOT terminate.*/
+CURLcode Curl_rand_hex(struct Curl_easy *data, unsigned char *rnd,
+                       size_t num);
 
 #endif /* HEADER_CURL_RAND_H */