avoid internal error on misconfigured executable
racket/runtime-path: fix ".ss" vs. ".rkt" search for `lib` paths

The package split exposed another place where searching
is needed, because the fallback in `collection-file-path`
to `collection-path` didn't work after the split.
slideshow: fix clipping for 4-up mode and for printing
fix custodian-related memory-management problem

The problem mainly affected `register-custodian-shutdown`
from `ffi/unsafe/custodian`, which is used by `math/bigfloat`
and `ffi/unsafe/com`.

When a value is registered with a custodian, the value is held
weakly, but the shutdown procedure is intended to be held
strongly. At the C API level, the data associated with a shutdown
function pointer is intended to be held strongly.

A custodian itself, however, is retained weakly by other custodians
in its family, so that custodians can be GCed and their elements
transferred to a parent custodian. Since the custodian itself may
be held only weakly, the callback & data in a custodian was effectively
held weakly --- which, in turn, can break assumptions in code such
as `ffi/unsafe/custodian` that expects strong references to prevent
finalizers from running.

Fix the problem by registering a reference to callback data as
data in a custodian's finalizer, which makes the data strongly
retained no matter how the custodian is retained.