pimd: Fix PIM VRF support (send register/register stop in VRF)

pimd: Fix PIM VRF support (send register/register stop in VRF)

In 946195391406269003275850e1a4d550ea8db38b and
8ebcc02328c6b63ecf85e44fdfbf3365be27c127, transmission of PIM register and
register stop messages was changed to use a separate socket. However, that
socket is not bound to a possible VRF, so the messages were sent in the
default VRF instead. Call vrf_bind() once after socket creation and when the
VRF is ready to ensure transmission in the correct VRF. vrf_bind() handles
the non-VRF case (i.e. VRF_DEFAULT) automatically, so it may be called
unconditionally.

Signed-off-by: Martin Buck <mb-tmp-tvguho.pbz@gromit.dyndns.org>