Temporarily fix breakage for nhc98.

A recent patch to System.IO introduced a cyclic dependency on Foreign.C.Error,
and also inadvertently dragged along System.Posix.Internals which has
non-H'98 layout, causing many build problems.  The solution for now
is to #ifndef __NHC__ all of the recent the openTempFile additions,
and mark them non-portable once again.  (I also took the opportunity
to note a number of other non-portable functions in their Haddock
comments.)