creating unique urls for binary packages

[matthew sporleder <msporleder%gmail.com@localhost>]

A few days ago we ran into a situation where some packages were
rebuilt but not given new names, causing pkgin and friends to be upset
about checksums of files not matching pkg_summary because
cdn.netbsd.org had the packages cached from the previous build.

There are a few ways to resolve this, but the easiest one is to have
the client (pkgin) append a unique identifier (like the build date) to
the package's url:
bash-2.05.2.13.tgz?BUILD_DATE=2017-01-07_23:23:31_+0000

when it fetched that package.

Thoughts?

Matt