lavu/random_seed: use arc4random() when available

arc4random() was designed as a superior interface for system random
number generation, designed for OpenBSD and subsequently incorporated by
other BSD's, Mac OS X, and some non-standard libc's. It is thus an improvement to
use it whenever available.

As a side note, this may or may not get included in glibc, and there is
a proposal to create a posix_random family based on these ideas:
http://austingroupbugs.net/view.php?id=859

.

Tested on Mac OS X.

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>