use which instead of type

This is for consistency with the rest of lxc, and also because type checks for
shell builtins, a behavior that we do not want in these cases. Ensure stderr
for which is redirected to /dev/null also.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>