Get Libevent's PRNG functioning under the linux sandbox

Libevent uses an arc4random implementation (I know, I know) to
generate DNS transaction IDs and capitalization.  But it liked to
initialize it either with opening /dev/urandom (which won't work
under the sandbox if it doesn't use the right pointer), or with
sysctl({CTL_KERN,KERN_RANDOM,RANDOM_UUIC}).  To make _that_ work, we
were permitting sysctl unconditionally.  That's not such a great
idea.

Instead, we try to initialize the libevent PRNG _before_ installing
the sandbox, and make sysctl always fail with EPERM under the
sandbox.