Low: named: Properly detect rndc stop failure

Launching rndc into the background meant we can't
access the return code easily without doing a wait
on the rndc pid.
Medium: named: Attempt to autogen /etc/rndc.key using rndc-confgen tool

This allows rndc to be used to stop and reload named without additional
configuration.
Merge pull request #263 from davidvossel/named_improvements

named agent improvements