pimd: Add null_register bit to pim_register_send

pimd: Add null_register bit to pim_register_send

When sending a pim register message to an RP, we need
the ability to set the null bit in the message.  This
adds the ability to do so.

Additionally we need to switch the ip_hdr to a const uint8_t *buf
because pim_register_send really shouldn't know or care about
the fact it's a 'struct ip'.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>