pg_basebackup fails if a data file is removed

When pg_basebackup copies data files, it does basically this:

> if (lstat(pathbuf, &statbuf) != 0)
> {
>     if (errno != ENOENT)
>         ereport(ERROR,
>                 (errcode_for_file_access(),
>                  errmsg("could not stat file or directory \"%s\": %m",
>                     pathbuf)));
>
>     /* If the file went away while scanning, it's no error. */
>     continue;
> }
 > ...
 > sendFile(pathbuf, pathbuf + basepathlen + 1, &statbuf);

There's a race condition there. If the file is removed after the lstat
call, and before sendFile opens the file, the backup fails with an
error. It's a fairly tight window, so it's difficult to run into by
accident, but by putting a breakpoint with a debugger there it's quite
easy to reproduce, by e.g doing a VACUUM FULL on the table about to be
copied.

A straightforward fix is to allow sendFile() to ignore ENOENT. Patch
attached.

- Heikki