MDEV-10541: Faking the version string only works with MariaDB-Clients

Our RPL_VERSION_HACK prefix caused MySQL clients to always report 5.5
major and minor versions, even if a specific fake version is passed via
my.cnf or command line parameters. When a specific version is requested,
don't employ the RPL_VERSION_HACK prefix within the server handshake
packet.

sql/mysqld.cc

@@ -630,6 +630,7 @@ Time_zone *default_tz;
 
 const char *mysql_real_data_home_ptr= mysql_real_data_home;
 char server_version[SERVER_VERSION_LENGTH], *server_version_ptr;
+bool using_custom_server_version= false;
 char *mysqld_unix_port, *opt_mysql_tmpdir;
 ulong thread_handling;
 
@@ -8960,6 +8961,7 @@ mysqld_get_one_option(int optid, const struct my_option *opt, char *argument)
     {
       strmake(server_version, argument, sizeof(server_version) - 1);
       set_sys_var_value_origin(&server_version_ptr, sys_var::CONFIG);
+      using_custom_server_version= true;
     }
 #ifndef EMBEDDED_LIBRARY
     else

sql/mysqld.h

@@ -544,6 +544,7 @@ extern const char *mysql_real_data_home_ptr;
 extern ulong thread_handling;
 extern "C" MYSQL_PLUGIN_IMPORT char server_version[SERVER_VERSION_LENGTH];
 extern char *server_version_ptr;
+extern bool using_custom_server_version;
 extern MYSQL_PLUGIN_IMPORT char mysql_real_data_home[];
 extern char mysql_unpacked_real_data_home[];
 extern MYSQL_PLUGIN_IMPORT struct system_variables global_system_variables;

sql/sql_acl.cc

@@ -12062,7 +12062,13 @@ static bool send_server_handshake_packet(MPVIO_EXT *mpvio,
     data_len= SCRAMBLE_LENGTH;
   }
 
-  end= strxnmov(end, SERVER_VERSION_LENGTH, RPL_VERSION_HACK, server_version, NullS) + 1;
+  /* When server version is specified in config file, don't include
+     the replication hack prefix. */
+  if (using_custom_server_version)
+    end= strnmov(end, server_version, SERVER_VERSION_LENGTH) + 1;
+  else
+    end= strxnmov(end, SERVER_VERSION_LENGTH, RPL_VERSION_HACK, server_version, NullS) + 1;
+
   int4store((uchar*) end, mpvio->auth_info.thd->thread_id);
   end+= 4;