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).
