MINOR: protocol: always initialize the receivers list on registration

MINOR: protocol: always initialize the receivers list on registration

Till now, protocols were required to self-initialize their receivers
list head, which is not very convenient, and is quite error prone.
Indeed, it's too easy to copy-paste a protocol definition and forget
to update the .receivers field to point to itself, resulting in mixed
lists. Let's just do that in protocol_register(). And while we're at
it, let's also zero the nb_receivers entry that works with it, so that
the protocol definition isn't required to pre-initialize stuff related
to internal book-keeping.