Add a Socket implementation which just holds an error message.

This isn't yet used, but I plan to use it in situations where you have
to report errors by returning a valid Socket on which the client wlil
call sk_socket_error, but in fact you notice the error _before_
instantiating your usual kind of Socket. The resulting Socket is
usable for nothing except reading out the error string and closing it.

[originally from svn r10065]

network.h

@@ -205,6 +205,12 @@ int net_service_lookup(char *service);
  */
 char *get_hostname(void);
 
+/*
+ * Trivial socket implementation which just stores an error. Found in
+ * errsock.c.
+ */
+Socket new_error_socket(const char *errmsg, Plug plug);
+
 /********** SSL stuff **********/
 
 /*