Changes for multi-OS basekits

* config.tcl now defines config parms for basekits for each OS flavor.
* env.tcl will now look for basekits for the current Tcl version and
the desired OS flavor on the local disk.
* If the current platform's flavor is desired, it looks where AT puts
it, as before.
* If it doesn't find it, or if it's a different flavor, looks in
~/.quill/basekits.