util: add function to generate random bytes

Add a function to fill a buffer with random bytes which uses a better
PRNG than random(). Use arc4random() if it's available on the system.
Fall back to reading from /dev/urandom, which should be available on
all currently supported systems.

util.h

@@ -144,4 +144,7 @@ extern int UTI_CreateDirAndParents(const char *path, mode_t mode, uid_t uid, gid
    permissions and its uid/gid must match the specified values. */
 extern int UTI_CheckDirPermissions(const char *path, mode_t perm, uid_t uid, gid_t gid);
 
+/* Fill buffer with random bytes */
+extern void UTI_GetRandomBytes(void *buf, unsigned int len);
+
 #endif /* GOT_UTIL_H */