MDEV-6344: mysqldump issues FLUSH TABLES, which gets written into binlog and replicated

Add a --gtid option (for compatibility, the original behaviour is preserved
when --gtid is not used).

With --gtid, --master-data and --dump-slave output the GTID position (the
old-style file/offset position is still output, but commented out). Also, a
CHANGE MASTER TO master_use_gtid=slave_pos is output to ensure a provisioned
slave is configured in GTID, as requested.

Without --gtid, the GTID position is still output, if available, but commented
out.

Also fix MDEV-6344, to avoid FLUSH TABLES getting into the binlog. Otherwise a
mysqldump on a slave server will silently inject a GTID which does not exist
on the master, which is highly undesirable.

Also fix an incorrect error handling around obtaining binlog position with
--master-data (was probably unlikely to trigger in most cases).