MINOR: protocol: export protocol definitions

MINOR: protocol: export protocol definitions

The various protocols were made static since there was no point in
exporting them in the past. Nowadays with QUIC relying on UDP we'll
significantly benefit from UDP being exported and more generally from
being able to declare some functions as being the same as other
protocols'.

In an ideal world it should not be these protocols which should be
exported, but the intermediary levels:
  - socket layer (sock.c only right now), already exported as functions
    but nothing structured at the moment ;
  - family layer (sock_inet, sock_unix, sockpair etc): already structured
    and exported
  - binding layer (the part that relies on the receiver): currently fused
    within the protocol
  - connectiong layer (the part that manipulates connections): currently
    fused within the protocol
  - protocol (connection's control): shouldn't need to be exposed
    ultimately once the elements above are in an easily sharable way.