Add defenses against pre-crash files to BufFileOpenShared().

Crash restarts currently don't clean up temporary files, as a debugging aid.
If a left-over file happens to have the same name as a segment file we're
trying to create, we'll just truncate and reuse it, but there is a problem:
BufFileOpenShared() determines how many segment files exist by trying to open
.0, .1, .2, ... until it finds no more files.  It might be confused by a junk
file that has the next segment number.  To defend against that, make sure we
always create a gap after the end file by unlinking the following name if it
exists.  Also make it an error to try to open a BufFile that doesn't exist
(has no segment 0), so as not to encourage the development of client code
that depends on an interface that we can't reliably provide.

Author: Thomas Munro
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/CAEepm%3D2jhCbC_GFQJaaDhWxLB4EXtT3vVd5czuRNaqF5CWSTog%40mail.gmail.com

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