Further thoughts about temp_file_limit patch.

Move FileClose's decrement of temporary_files_size up, so that it will be
executed even if elog() throws an error.  This is reasonable since if the
unlink() fails, the fact the file is still there is not our fault, and we
are going to forget about it anyhow.  So we won't count it against
temp_file_limit anymore.

Update fileSize and temporary_files_size correctly in FileTruncate.
We probably don't have any places that truncate temp files, but fd.c
surely should not assume that.