detect changes to "links.rktd" by content instead of timestamp

The 1-second granularity of filesystem timestamps is not
good enough to deetct changes to the file --- especially
when `raco pkg' installs links and runs `raco setup' (in
the same Rcket process), in which case a newly installed link
might not be detected.

The longer-term repair is probably to add file-changed
events to Racket (based on `inotify' and similar interfaces
provided by OSes).