unshare: fix -r

unshare: fix -r

Calling unshare(2) immediately puts us in the new namespace
with the "overflow" user and group ID. By calling geteuid()
and getegid() in handle_r() after calling unshare(), we try
to map that to root, which Linux refuses to let us do.

What we really want to map to root is the caller's uid/gid
in the original namespace. So we have to save them before
calling unshare().