bgpd/pimd: fix zAPI parsing

bgpd/pimd: fix zAPI parsing

Commit c99f3481a598 has changed the API. Now, the vrfid has been added in
the header, thus we must read it before parsing the rest of the message.

To ease code maintenance, let's add a new function to read a zAPI header.

Fixes: c99f3481a598 ("*: add VRF ID in the API message header")
Reported-by: Martin Winter <mwinter@opensourcerouting.org>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Donald Sharp <sharpd@cumulusnetworks.com>
Tested-by: Martin Winter <mwinter@opensourcerouting.org>