Add -u option to make us setuid()

If you specify -u, we will setuid() and setgid() to that username and her group
after we bind & chroot. This is effectively the same as dropping capabilities,
but it also works on non-Linux platforms.
OSX: Print error if we're root and don't setuid().
Merge pull request #3 from jorgenpt/add-setuid

Add setuid support