Prevent client use of LOAD DATA INFILE, SELECT INTO OUTFILE statements and LOAD_FILE() function

In cloud environments, users have no access to the server filesystem, so there should be a way to disable the SELECT INTO OUTFILE and LOAD DATA INFILE statements and the LOAD_FILE() function, as there will never be a legitimate use of them.

In order to prevent MySQL clients from interacting with the server filesystem, the secure-file-priv server option will be extended so that it can be used without an argument. When used with no argument, the LOAD_FILE() function will always return NULL. The LOAD DATA INFILE and SELECT INTO OUTFILE statements will fail with the following error: "The MySQL server is running with the --secure-file-priv option so it cannot execute this statement". LOAD DATA LOCAL INFILE is not affected by the --secure-file-priv option and will still work when it's used without an argument.