Switch user ID/root directory with -Z before opening savefiles for output.

Switch user ID/root directory with -Z before opening savefiles for output.

In addition to Paul's change, I added a comment explaining why we do
this, and explaining that doing so after opening the first savefile
doesn't help with subsequent savefiles, so you'll have to come up with a
better fix if you want the savefiles opened by the original UID or
outside the chroot.

Reviewed-By: Guy Harris <[email protected]>