Backport mysql-test/t/myisam_explain_non_select_all.test from mysql-5.6
- the .result file was modified because MariaDB choses different 
  query plans in a number cases. Also, we don't have some of the 
  "incorrect EXPLAIN output" bugs that they still have.

The .test file and includes were taken verbatim with one 
exception: two tests were disabled with --disable parsing:

1. @@sql_safe_updates is not enforced EXPLAINs of multitable 
   updates. In MariaDB, the execution itself will produce 
   ER_UPDATE_WITHOUT_KEY_IN_SAFE_MODE, but EXPLAIN won't.
2. Their case #71 hits some old bug in MyISAM (See their comments
   in explain_non_select.inc for details).

- Produce correct `key_len` when type=index.

- Produce correct #rows for ORDER BY ... LIMIT N queries that take advantage of 
  ordered index read to read only N rows.

- Generate correct contents of `Extra` column for UPDATEs/DELETEs that use quick selects
- UPDATEs with used_key_is_modified=true will show "Using buffer"

- Update test results after last few csets
- Generate correct value for `possible_keys` column for single table UPDATE/DELETE.

- if EXPLAIN DELETE prints "Deleting all rows", it should show the 
  expected number of rows in the rows column.

- Add support for EXPLAIN INSERT.

- Better EXPLAIN-saving methods for quick selects

- Run probes like MYSQL_INSERT_SELECT_START or 
  MYSQL_MULTI_DELETE_START for EXPLAIN, too.
  We should run them, because 1. EXPLAIN SELECT does it, and 
  2. MySQL also does it.

- Handle the case when EXPLAIN UPDATE/DELETE has pruned away all partitions.

- Address review feedback: more renames

- Address review feedback: rename files

- Address review feedback: rename nearly any name used by the new EXPLAIN code.

- Added a testcase

- Make EXPLAIN {PARTITIONS,EXTENDED} {UPDATE,DELETE} work.

- Make EXPLAIN UPDATE/DELETE work inside SPs
- Return correct error code from mysql_delete()
- EXPLAIN <multi-DELETE> will create a multi_delete object (as it 
  affects the optimization). select_result will be only used for 
  producing EXPLAIN output.

- Query plan footprint (in new terms, "EXPLAIN structure") should always keep 
  a copy of key_name.  This is because the table might be a temporary table which
  may be already freed by the time we use query plan footprint.

- If we're running explain with flags, use the same set of flags to make EXPLAIN columns 
  and contents.

- Add EXPLAIN output print out for INSERT/REPLACE ... SELECT

- update mysqld--help.result

- Don't forget to delete the query plan footprint when 
  the query wasn't printed into slow query log for some reason
- ALso removed some garbage code.

- Don't print a plan when the statement didn't produce it
- Also, add first testcase. We can't check the EXPLAIN from the slow log itself, though.

- Initial implementation. 

- Make QPF structures store data members, not strings. 
  This is not fully possible, because table names (and hence 
  key names, etc) can be deleted, and we have to store strings.

- Post-merge fixes (conflict with DELETE .. RETURNING)
- Add a testcase with EXPLAIN ... DELETE ... RETURNING

- Merge with current 10.0-base

- Merge with current 10.0-base

Implement @@gtid_binlog_state. This is the internal state of the binlog
(most recent GTID logged for every domain_id and server_id). This allows
to save the state before RESET MASTER and restore it afterwards.

The ignored events are not written to the relay log, but instead a fake
Rotate event is generated to handle update of position.

Extend this for Gtid so we similarly generate a fake Gtid_list event
to update the GTID position.

Also fix an unrelated test issue that got triggered by the added test cases.

- Let sp_get_flags_for_command() set sp_head::MULTI_RESULTS for DELETE ... RETURNING, 
  like it does for all statements that return a resultset.

  
Currently the loose scan code in opt_range.cc considers all indexes as
possible for the access method. Due to inexact statistics it may happen
that a loose scan is selected over a unique index.
  
This is clearly wrong since a "loose scan" over a unique index will read
the same keys as a direct index scan, but the loose scan has more overhead.
  
This task adds a rule to skip unique indexes for loose scan.

It was fixed as merge changes for MDEV-4811.