Yet another complete rewrite. Hopefully the *last* complete rewrite of

this command for a while; this implements roughly the plan cooked up by
Guido, Fred, and me.  Seems to strike a nice balance between usability in
the common cases (just set one option), expandability for more types of
files to install in future, and customizability of installation
directories.

This revision isn't completely working: standard and alternate
installations work fine, but there are still some kinks to work out of
customized installations.