Upgrade formrdesc() so that it can correctly initialize the tupledesc

(rd_att) field of a nailed-in-cache relcache entry.  This fixes the bug
reported by Alvaro 8-Dec-2004; I believe it probably also explains
Grant Finnemore's report of 10-Sep-2004.

In an unrelated change in the same file, put back 7.4's response to
failure to rename() the relcache init file, ie, unlink the useless
temp file.  I did not put back the warning message, since there might
actually be some reason not to have that.