SYSV: Implement downloading and installing SYSV packages from HTTP

pkgadd has different syntax for different sources (datastream and file
system format) which makes using a single pkgtool variable difficult.
Also, SYSV packages in datastream format don't necessarily have uniform
names.  Therefore, use the existing 'simplename' attribute to specify
the datastream file name.
We should probably add some logging here to let people know what's happening; otherwise, this (potentially time-consuming) step could be mystifying.