Store protocol by id, add lookup function for name

Store protocol by id, add lookup function for name

We want to use the protocol id for lookup in the GSocketConnection
code, so we expose it. We also make GSocket store the protocol
as an int for less memory use and to allow platform specific protocols
to be specified.

Also added g_socket_protocol_id_lookup_by_name() to allow the higher
level code to specify the name by string, and g_socket_get_protocol_name()
to get it.