initdb -S should now have an explicit check that $PGDATA is valid.

The fsync code from the backend essentially assumes that somebody's already
validated PGDATA, at least to the extent of it being a readable directory.
That's safe enough for initdb's normal code path too, but "initdb -S"
doesn't have any other processing at all that touches the target directory.
To have reasonable error-case behavior, add a pg_check_dir call.
Per gripe from Peter E.