sysdep: Add wrapper to get random bytes - update

Simplify the code and fix an issue with getentropy() return value.

lib/birdlib.h

@@ -192,8 +192,7 @@ asm(
 /* Pseudorandom numbers */
 
 u32 random_u32(void);
-int random_bytes(char *buf, size_t size);
-void random_close(void);
 void random_init(void);
+void random_bytes(void *buf, size_t size);
 
 #endif