On Windows, getpid() is _getpid(), and requires that we first include
<process.h>. arc4random.c previously didn't know that.
Actually, I question whether arc4random needs to do its getpid() tricks
on Windows. They exist only so that we remember to re-seed the ARC4
cipher whenever we fork... but Windows has no fork(), so I think we're
in the clear.
Previously, evdns was at the mercy of the user for providing a good
entropy source; without one, it would be vulnerable to various
active attacks.
This patch adds a port of OpenBSD's arc4random() calls to Libevent
[port by Chris Davis], and wraps it up a little bit so we can use it
more safely.
