Use stat(2) to probe for existing xlog segments in InstallXLogFileSegment,

Use stat(2) to probe for existing xlog segments in InstallXLogFileSegment,
rather than actually opening the files.  This eliminates some corner cases
where the file indeed exists but open() fails for another reason, such
as being out of file descriptors.  The net reliability gain is probably
tiny, since xlog.c is full of other file open calls that will elog(PANIC)
if they fail for any reason; but this specific failure mode has been
observed in the field, so we may as well fix it.