emacs.samples: Reliably override ".dir-locals.el".

Back-patch to 9.4, where .dir-locals.el was introduced.
Adjust 9.4 release notes.

Back-patch to 9.4.
Harden pg_filenode_relation test against concurrent DROP TABLE.

Per buildfarm member prairiedog.  Back-patch to 9.4, where the test was
introduced.

Reviewed by Tom Lane.