rand()/srand() are not cryptographically secure PRNGs. Some systems

rand()/srand() are not cryptographically secure PRNGs.  Some systems
have the much better suited arc4random() family of functions; there's
no reason to not use it if it is available.  Make m_rand() just a
wrapper around arc4random_buf() in that case.  (There's no need to
ever seed it manually.)

As a bonus, silences some warnings on OpenBSD.