RFC: New pexecute interface

[Zack Weinberg]

The interface looks sound to me with one exception: it's not safe to
conflate !-pipe with -save-temps, because that opens up the
possibility of a tempfile race -- if an attacker sees that the
compiler is producing /tmp/ccQWERTY.s, then they should not be able to
predict that the assembler will produce /tmp/ccQWERTY.o.

Also, why the parentheses around the numbers?

zw