bfdd: make bfd_recv_* more generic

bfdd: make bfd_recv_* more generic

We'll reuse these functions later to receive echo packets, so we need
to despecialize them first. Another improvement here is that we are
going to remove the global buffer variable and opt to use the stack.

Signed-off-by: Rafael Zalamena <rzalamena@opensourcerouting.org>