Fix for BUG#3829 "Setting server_id on fly doesn't allow replication to start"

(fix by our Harrison Fisk): when one does SET GLOBAL SERVER_ID=x, we must set
server_id_supplied to 1.


sql/mysql_priv.h:
  server_id_supplied must be here to be visible in set_var.cc
sql/mysqld.cc:
  rephrasing warnings when server id is not set explicitely.
sql/set_var.cc:
  when one does SET GLOBAL SERVER_ID=x; it should be considered as explicitely setting
  the server id, so do server_id_supplied=1.
sql/slave.cc:
  Correcting wrong comment