Always print slave host in SHOW SLAVE HOSTS

Since the client host can be extracted from the network connection, it can
always be printed. This makes it easier to find out where a slave is
replicating from. It could also be used to automatically discover slaves
that are replicating from a master.

mysql-test/suite/rpl/r/rpl_show_slave_hosts.result

@@ -10,11 +10,11 @@ connection master;
 SHOW SLAVE HOSTS;
 Server_id	Host	Port	Master_id
 3	slave2	SLAVE_PORT	1
-2		SLAVE_PORT	1
+2	localhost	SLAVE_PORT	1
 connection slave2;
 include/stop_slave_io.inc
 connection master;
 SHOW SLAVE HOSTS;
 Server_id	Host	Port	Master_id
-2		SLAVE_PORT	1
+2	localhost	SLAVE_PORT	1
 include/rpl_end.inc

sql/repl_failsafe.cc

@@ -147,6 +147,9 @@ int THD::register_slave(uchar *packet, size_t packet_length)
   if (!(si->master_id= uint4korr(p)))
     si->master_id= global_system_variables.server_id;
 
+  if (!*si->host)
+    ::strmake(si->host, main_security_ctx.host_or_ip, sizeof(si->host));
+
   unregister_slave();
   mysql_mutex_lock(&LOCK_thd_data);
   slave_info= si;