bug#6900: mktemp: want option to make a fifo

[John Reiser]

>   (echo quit 1 | cmd1 3<&0 <&4) 4<&0 |
>   (:           | cmd2 3<&0 <&4) 4<&0

When adopting this style, then remember to consider the possibility of
closing the "extra" file descritpor (fd 4 here) to reduce the number
of open files.  If there are no 'trap', then also remember to consider
the possibility of using 'exec' to reduce the number of processes created
['fork' or 'clone' system calls] and their lifetimes:

   (exec echo quit 1 | exec cmd1 3<&0 <&4 4<&-) 4<&0 |
   (     :           | exec cmd2 3<&0 <&4 4<&-) 4<&0

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