machinectl: implement "bind" command to create additional bind mounts from host to container during runtime
update TODO
util: in make_stdio() use dup2() rather than dup3()

dup3() allows setting O_CLOEXEC which we are not interested in. However,
it also fails if called with the same fd as input and output, which is
something we don't want. Hence use dup2().

Also, we need to explicitly turn off O_CLOEXEC for the fds, in case the
input fd was O_CLOEXEC and < 3.
machinectl: add new commands for copying files from/to containers