file/gunzip: fix problem with lookahead handling

If less than the lookahead bytes have been pulled in, then the
old code would attempt to read a negative number of bytes.
raco setup: put launchers for user collections in user space

Also, remove executables (in full setup or "tidy" mode) when the
corresponding collections are removed.

As a result of these changes, `raco pkg' puts launchers from user-scoped
packages in user space and cleans up launchers from removed packages.

There's no attempt to include "launchers.rktd" in a distribution.
That should be ok: having an entry in "launchers.rktd" just
makes a launcher a candidate for removal. Including "launchers.rktd"
would be a hassle for many reasons, including that the initial set
of launchers is platform-specific.