dup2 requires a include. Fixes the following when diffing config.log
when testing with a stricter compiler:
```
-warning: call to undeclared function 'dup2'; ISO C99 and later do not support
implicit function declarations [-Wimplicit-function-declaration]
+error: call to undeclared function 'dup
> ...in posix mode, fname must be a valid shell name and may not be the name
> of one of the POSIX special builtins.
> In default mode, a function name can be any unquoted shell word that does
> not contain $. ...
>
I'm guessing the intention is that it shouldn't contain any expansions, so
it als
