make in-place: keep non-default `PKGS' value in "racket/etc/link-pkgs.rktd"

This change makes a `PKGS=...' setting sticky for an in-place build via
`make', `make in-place', `make pkg-links', etc.
pkg/strip: allow any info library path
fix some clang warnings

The "if (pi->unwind_info);" change might be a bug fix.
fix exception handler inside links-reading code
add distro-builds/manage-snapshots