Added support for --skip-secure-file-priv

This works the same as secure-file-priv="", but is more obvious way to
turn of secure-file-priv.

sql/mysqld.cc

@@ -8261,6 +8261,13 @@ mysqld_get_one_option(const struct my_option *opt, const char *argument,
   case OPT_SERVER_ID:
     ::server_id= global_system_variables.server_id;
     break;
+  case OPT_SEQURE_FILE_PRIV:
+    if (argument == disabled_my_option)
+    {
+      my_free(opt_secure_file_priv);
+      opt_secure_file_priv= 0;
+    }
+    break;
   case OPT_LOWER_CASE_TABLE_NAMES:
     lower_case_table_names_used= 1;
     break;

sql/mysqld.h

@@ -842,6 +842,7 @@ enum options_mysqld
   OPT_MYSQL_COMPATIBILITY,
   OPT_TLS_VERSION,
   OPT_MYSQL_TO_BE_IMPLEMENTED,
+  OPT_SEQURE_FILE_PRIV,
   OPT_which_is_always_the_last
 };
 #endif

sql/sys_vars.cc

@@ -3389,7 +3389,7 @@ static Sys_var_charptr_fscs Sys_secure_file_priv(
        "Limit LOAD DATA, SELECT ... OUTFILE, and LOAD_FILE() to files "
        "within specified directory",
        PREALLOCATED READ_ONLY GLOBAL_VAR(opt_secure_file_priv),
-       CMD_LINE(REQUIRED_ARG), DEFAULT(0));
+       CMD_LINE(REQUIRED_ARG, OPT_SEQURE_FILE_PRIV), DEFAULT(0));
 
 static bool check_server_id(sys_var *self, THD *thd, set_var *var)
 {