Setup download adjustments

[Max Bowsher]

1) Concious of the fact that http://cygwin.com/setup-snapshots/ generally 
contains release candidates, old versions, and source, but no snapshots I've 
renamed it to http://cygwin.com/setup/.

2) Disliking the way setup.exe provides a severe lack of indication that 
source-only packages are source-only, and finding it a little bizarre that 
the source and binary of setup.exe are distributed in different ways, I've 
moved the setup source from the package area to the above URL, too. (And 
updated http://sources.redhat.com/cygwin-apps/setup.html).

[Attn CGF: Could you drop the remnant stub entry from setup.ini, please.]

Max.