These files are deleted but not yet gone from the filesystem. Operations
on them will return ERROR_DELETE_PENDING.

With this we start treating that as ENOENT, meaning files does not
exist (which is the state it will soon reach). This should be safe in
every case except when we try to recreate a file with exactly the same
name. This is an operation that PostgreSQL does very seldom, so
hopefully that won't happen much -- and even if it does, this treatment
should be no worse than treating it as an unhandled error.

We've been un able to reproduce the bug reliably, so pushing this to
master to get buildfarm coverage and other testing. Once it's proven to
be stable, it should be considered for backpatching.

Discussion: https://postgr.es/m/20160712083220.1426.58667%40wrigleys.postgresql.org

Patch by me and Michael Paquier