- Fix a problem with EXPLAIN multi_table UPDATE: 
  = Do use multi_update object, because multi_update::prepare() does
    various setup, e.g. it disables index-only for the tables to be updated.
  = Protect multi_update::prepare() from being invoked multiple times. 
    If the query has subqueries, they may try to invoke it, for some reason.

- eliminate join_save_qpf() function.

Update the SHOW EXPLAIN code to work with the 
new architecture (part#1):
Before, SHOW EXPLAIN operated on real query plan structures, 
which meant it had to check when SELECTs are created/deleted.
SELECTs would call apc_target->enable() when they got a query 
plan and disable() when their query plan was deleted.

Now, Explain data structure becomes available at once (and we
call apc_target->enable()) and then it stays until it is deleted
(when that happens, we call apc_target->disable()).

- Fix EXPLAIN INSERT DELAYED ... : do call end_delayed_insert().

- When INSERT catches an error, it should not attempt to send parts of 
  EXPLAIN output.

- Don't save UNION's EXPLAIN data if optimization failed with 
  an error. We could end up saving incomplete plan, which will 
  cause a crash when we attempt to print it.

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