MDEV-18544 "missing required privilege PROCESS on *.*" using mariabackup for SST

If required privilege is missing, dump the output from "SHOW GRANTS"
into mariabackup log.

This will help troubleshooting, and make the bug reproducible.

extra/mariabackup/xtrabackup.cc

@@ -5820,6 +5820,13 @@ check_all_privileges()
 
 	if (check_result & PRIVILEGE_ERROR) {
 		mysql_close(mysql_connection);
+		msg("Current privileges, as reported by 'SHOW GRANTS': ");
+		int n=1;
+		for (std::list<std::string>::const_iterator it = granted_privileges.begin();
+			it != granted_privileges.end();
+			it++,n++) {
+				msg("  %d.%s", n, it->c_str());
+		}
 		die("Insufficient privileges");
 	}
 }

mysql-test/suite/mariabackup/backup_grants.result

 CREATE user backup@localhost;
 FOUND 1 /missing required privilege RELOAD/ in backup.log
 FOUND 1 /missing required privilege PROCESS/ in backup.log
+FOUND 1 /GRANT USAGE ON/ in backup.log
 GRANT RELOAD, PROCESS on *.* to backup@localhost;
 DROP USER backup@localhost;

mysql-test/suite/mariabackup/backup_grants.test

@@ -18,7 +18,8 @@ let SEARCH_FILE=$MYSQLTEST_VARDIR/tmp/backup.log;
 --source include/search_pattern_in_file.inc
 --let SEARCH_PATTERN=  missing required privilege PROCESS
 --source include/search_pattern_in_file.inc
-
+--let SEARCH_PATTERN= GRANT USAGE ON
+--source include/search_pattern_in_file.inc
 # backup succeeds with RELOAD privilege
 GRANT RELOAD, PROCESS on *.* to backup@localhost;
 --disable_result_log