Remove protocol names, instead use an enum with common protocols

The whole protocol name thing is pretty weird. The getprotobyname functions
seem to only specify one mapping for name <-> ids, so all families/types
must use the same values. Plus the values used for the protocols are
standardized by IANA, so are always the same.

So, we drop using names for protocols, intead introducing an enum with
a few commonly availible and used protocols.

gio/gioenums.h

@@ -556,6 +556,30 @@ typedef enum
   G_SOCKET_MSG_DONTROUTE = GLIB_SYSDEF_MSG_DONTROUTE
 } GSocketMsgFlags;
 
+/**
+ * GSocketProtocol:
+ * @G_SOCKET_PROTOCOL_UNKNOWN: The protocol type is unknown
+ * @G_SOCKET_PROTOCOL_DEFAULT: The default protocol for the family/type
+ * @G_SOCKET_PROTOCOL_TCP: Tcp over IP
+ * @G_SOCKET_PROTOCOL_UDP: UDP over IP
+ * @G_SOCKET_PROTOCOL_SCTP: SCTP over IP
+ *
+ * A protocol identifier is specified when creating a #GSocket, which is a family/type
+ * specific identifier, where 0 means the default protocol for the particular family/type.
+ * This enum contains a set of commonly availible and used protocols. You can also
+ * pass any other identifiers handled by the platform in order to use protocols not
+ * listed here.
+ *
+ * Since: 2.22
+ */
+typedef enum {
+  G_SOCKET_PROTOCOL_UNKNOWN = -1,
+  G_SOCKET_PROTOCOL_DEFAULT = 0,
+  G_SOCKET_PROTOCOL_TCP     = 6,
+  G_SOCKET_PROTOCOL_UDP     = 17,
+  G_SOCKET_PROTOCOL_SCTP    = 132
+} GSocketProtocol;
+
 G_END_DECLS
 
 #endif /* __GIO_ENUMS_H__ */