When copying from an active database tree, it's possible for files to be
deleted after we see them in a readdir() scan but before we can open them.
(Once we've got a file open, we don't expect any further errors from it
getting unlinked, though.)  Tweak RecursiveCopy so it can cope with this
case, so as to avoid irreproducible test failures.

Back-patch to 9.6 where this code was added.  In v10 and HEAD, also
remove unused "use RecursiveCopy" in one recovery test script.

Michael Paquier and Tom Lane

Discussion: https://postgr.es/m/24621.1504924323@sss.pgh.pa.us