Provide the correct POSIX return value for POSIX::dup2() on Win32.

Provide the correct POSIX return value for POSIX::dup2() on Win32.

Microsoft, in their wisdom, chose to ignore the POSIX spec when implementing
their dup2(), and have theirs return 0 on success, instead of the file
descriptor. It seems that no other vendor is this, um, "special", so code
the exception directly, as we don't run Configure on Win32, so there's little
point probing for this.

This resolves RT #98912.