snapshot management: add "current" link
move configuration-file info from "INSTALL.txt" to "pkgs/distro-build"
fix GC bug in FFI

A foreign procedure's data could get GCed before the procedure
was called, if the procedure is not otherwise reachable.

(Bug found by running tests in GC-stress mode.)
fix an FFI test case
fix GC bug

This is a variant of the bug fixed in
49fbca5b1830bc4bddb8522d7647518227a32dd0,
where a GC-notification callback still
could cause problems in a GC during a finalization
callback.

(Bug found by running tests in GC-stress mode.)