Use urandom in dhcp, and where configurable in package builds

[bureado]

This is a very minor suggestion PR.

The first change makes ISC DHCP use /dev/urandom instead of its default. It also adds /dev/urandom to a small helper function in the ISC BIND spec.

It doesn't look like the version of bind9 being shipped in AL3 supports specifying the randomdev anymore, so I didn't make any other changes there.

It's possible other packages shipped in AL2+ support configuring the PRNG in configure, but I don't have a readily available list of them. Some packages seem to perform PRNG autodetection on configure, or at runtime. The best way to determine it would be to profile package builds (assuming tests hit /dev/random) or production.

I have not built and tested the new DHCP package since I lack the test harness and infrastructure to test dhclient, but I'm leaving it here as a suggestion.

Feel free to ack for consideration, close it, and bring the change back as part of a future commit, or keep it open dontmerge while you investigate other specs that could be changed to use urandom.