Optimize arc4random_uniform() (by syncing with OpenBSD implementation)

Optimize arc4random_uniform() (by syncing with OpenBSD implementation)

1. In d4de062, in Feb 2010, libevent adopted OpenBSD implementation of
   arc4random_uniform.
2. In
   https://github.com/openbsd/src/commit/728918cba93e0418bea2a73c9784f6b80c2a9dbd,
   in Jun 2012, OpenBSD improved their implementation to be faster, by
   changing arc4random_uniform() to calculate ``2**32 % upper_bound'' as
   ``-upper_bound % upper_bound''.

Alternatively we can simply remove arc4random_uniform() since it is not
used by libevent anyway, but let's just sync the header for now.