Manual pages: unshare.1: fix examples, part 2

Manual pages: unshare.1: fix examples, part 2

The explanation of the --kill-child example was quite confused and
also the example shell demo was broken because of quoting issues.

It is not the case that the *children* of 'program' would adopted by
init, but rather that 'program' itself (which would be running as PID
1 inside the namespace and is a child of 'unshare') would be adopted
by init.

Rework the --kill-child example. Add a lot more explanation, and
expand the example shell session to give the reader a much better
picture of what is going on.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>