lib, vtysh: fix inconsistent VRF commands in vtysh

lib, vtysh: fix inconsistent VRF commands in vtysh

* Only zebra and pimd call vrf_cmd_init(), so these are the only daemons
  that should receive VRF commands from vtysh;
* "netns NAME" and "no netns NAME" are available only in zebra, write
  custom DEFSHs in vtysh to make it aware of that;
* Remove the "no vrf NAME" definition from vtysh.c and expose the
  original command to vtysh by converting the DEFUN_NOSH to a simple
  DEFUN. This command doesn't change the vty node so there's no need to
  special case it.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>