MINOR: protocol: support a generic way to call getsockopt() on a connection

MINOR: protocol: support a generic way to call getsockopt() on a connection

It's regularly needed to call getsockopt() on a connection, but each
time the calling code has to do all the job by itself. This commit adds
a "get_opt()" callback on the protocol struct, that directly calls
getsockopt() on the connection's FD. A generic implementation for
standard sockets is provided, though QUIC would likely require a
different approach, or maybe a mapping. Due to the overlap between
IP/TCP/socket option values, it is necessary for the caller to indicate
both the level and the option. An abstraction of the level could be
done, but the caller would nonetheless have to know the optname, which
is generally defined in the same include files. So for now we'll
consider that this callback is only for very specific use.

The levels and optnames are purposely passed as signed ints so that it
is possible to further extend the API by using negative levels for
internal namespaces.