Embrace the platform string, part I

Previously, I was using the 'os flavor' as a proxy for the
platform::identify string.  This doesn't work very well.  The 'os
flavor' is about platform conventions; the platform::identify string is
about hardware and OS architectures.

As of now:

* In project.quill, the "app" statement has a -exetype that can be
"kit" or "exe".  This replaces -apptypes.
* When building an app, the executable name always includes the project
version and (for "exe" apps) the platform::identify string.
* When checking dependencies, only the base kits for the current
platform are checked.  For cross-platform, we'll do it differently.

The 'dist' mechanism, and the 'env' and 'config' code related to base
kits still needs to be updated.
Embrace the platform string, part II

A "dist" set name can now include "%platform"; this will add the
platform::identify string to the created distribution .zip file name.
Also, added "%apps" as a pattern to the set of dist file patterns; it
pulls in all applications for the current platform.
Embrace the platform string, part III

The env and config modules now look just for the base kits for the
current platform.  'kite build all' -platform is a separate case.