Hacked things up a bit so that configuration variables are expanded

Hacked things up a bit so that configuration variables are expanded
in command-line options, and in two phases at that: first, we expand
'install_base' and 'install_platbase', and then the other 'install_*'
options.  This lets us do tricky stuff like
    install --prefix='/tmp$sys_prefix'
...oooh, neat.

Simplified 'select_scheme()' -- it's no longer responsible for expanding
config vars, tildes, etc.

Define installation-specific config vars in 'self.config_vars', rather than
in a local dictionary of one method.  Also factored '_expand_attrs()' out
of 'expand_dirs()' and added 'expand_basedirs()'.

Added a bunch of debugging output so I (and others) can judge the
success of this crazy scheme through direct feedback.