unshare: allow to set a new root

unshare: allow to set a new root

This patch instroduces two new parameters to set the new
root and the new working directory in this new root.

This allows to combine "unshare chroot" in one command,
and doing like this the /proc filesystem is correctly
mounted in the new root with "--mount-proc".

The new parameters are -R, --root and -w, --wd. The names
are the same as for nsenter, except for "-r" that is already
used by "--map-root-user" and replaced by "-R".

Signed-off-by: Laurent Vivier <laurent@vivier.eu>