The 'slave_patternload_file' is assigned to the real path of the load data file 
when initializing the object of Relay_log_info. But the path of the load data
file is not formatted to real path when executing event from relay log. So the 
error will be encountered if the path of the load data file is a symbolic link.

Actually the global 'opt_secure_file_priv' is not formatted to real path when 
loading data from file. So the same thing will happen too.

      
To fix these errors, the path of the load data file should be formatted to 
real path when executing event from relay log. And the 'opt_secure_file_priv' 
should be formatted to real path when loading data infile.


mysql-test/suite/rpl/r/rpl_loaddata_symlink.result:
  Test result for bug#43913.
mysql-test/suite/rpl/t/rpl_loaddata_symlink-master.sh:
  Added the test file to create a link from $MYSQLTEST_VARDIR/std_data 
  to $MYSQLTEST_VARDIR/std_data_master_link
mysql-test/suite/rpl/t/rpl_loaddata_symlink-slave.sh:
  Added the test file to create a link from $MYSQLTEST_VARDIR/std_data 
  to $MYSQLTEST_VARDIR/std_data_slave_link
mysql-test/suite/rpl/t/rpl_loaddata_symlink.test:
  Added the test file to verify if loading data infile will work fine 
  if the path of the load data file is a symbolic link.
sql/rpl_rli.cc:
  Added call 'my_realpath' function for avoiding sometimes the 'fn_format' 
  function can't format real path rightly.