Currently a package's cond-expand contains the symbol of the target
implementation and optionally more from config file. Execute a
command (once) on target implementation to add their full feature list,
making it available for each package to use.
All of these Schemes are tested. Larceny is just too annoying to get the
feature list (no one-liner, and it could take a while) so Larceny stays
the current behavior.
There is a small unrelated change here: the gosh command to get
version. We don't need to call (exit), if stdin is closed properly (it
should) then gosh should exit regardless.
