MINOR: protocols: add a new protocol type selector

MINOR: protocols: add a new protocol type selector

The protocol selection is currently performed based on the family,
control type and socket type. But this is often not enough, as both
only provide DGRAM or STREAM, leaving few variants. Protocols like
SCTP for example might be indistinguishable from TCP here. Same goes
for TCP extensions like MPTCP.

This commit introduces a new enum proto_type that is placed in each
and every protocol definition, that will usually more or less match
the sock_type, but being an enum, will support additional values.