Communicate server featureset (and changes) across server links.

Previously various information was only available for directly attached
servers, since it is communicated via PROTOCTL.
Now, we will also communicate information about leafs behind us.
IRCOps can use the /SINFO command to see these server features.
Services codes don't need to do anything, or at least are not expected
to do anything. They can still receive the information and do something
with it, of course...
Read the following technical documentation for full information,
as it will outline very specific rules for using the command S2S:
https://www.unrealircd.org/docs/Server_protocol:SINFO_command

include/struct.h

@@ -696,8 +696,9 @@ struct Server {
 	char 		*up;		/* uplink for this server */
 	char 		by[NICKLEN + 1];
 	ConfigItem_link *conf;
-	TS   		timestamp;		/* Remotely determined connect try time */
-	long		 users;
+	TS   		timestamp;	/* Remotely determined connect try time */
+	long		users;
+	TS		boottime;	/* Startup time of server */
 #ifdef	LIST_DEBUG
 	aClient *bcptr;
 #endif
@@ -706,9 +707,11 @@ struct Server {
 		unsigned server_sent:1;		/* SERVER message sent to this link? (for outgoing links) */
 	} flags;
 	struct {
+		char *usermodes;
 		char *chanmodes[4];
 		int protocol;
 		char *software;
+		char *nickchars;
 	} features;
 };