sysdep: Add wrapper to get random bytes

Add a wrapper function in sysdep to get random bytes, and required checks
in configure.ac to select how to do it. The configure script tries, in
order, getrandom(), getentropy() and reading from /dev/urandom.

sysdep/unix/main.c

@@ -867,6 +867,7 @@ main(int argc, char **argv)
   parse_args(argc, argv);
   log_switch(1, NULL, NULL);
 
+  random_init();
   net_init();
   resource_init();
   timer_init();