Fix for BUG#15110: mysqldump --triggers: does not include DEFINER clause

There are two main idea of this fix:
  - introduce a common function for server and client to split user value
    (<user name>@<host name>) into user name and host name parts;
  - dump DEFINER clause in correct format in mysqldump.

include/mysql_com.h

@@ -27,6 +27,14 @@
 #define SERVER_VERSION_LENGTH 60
 #define SQLSTATE_LENGTH 5
 
+/*
+  USER_HOST_BUFF_SIZE -- length of string buffer, that is enough to contain
+  username and hostname parts of the user identifier with trailing zero in
+  MySQL standard format:
+  user_name_part@host_name_part\0
+*/
+#define USER_HOST_BUFF_SIZE HOSTNAME_LENGTH + USERNAME_LENGTH + 2
+
 #define LOCAL_HOST	"localhost"
 #define LOCAL_HOST_NAMEDPIPE "."