"DECLARE CURSOR FOR SHOW ..." is a syntax that currently appears to work,
but is untested for some SHOW commands and does not work for other SHOW
commands.

Since this is an un-intended feature that leaked as a result of a coding bug
(in the parser grammar), the correct fix is to fix the grammar to not accept
this construct.

In other words, "DECLARE CURSOR FOR SHOW <other commands that don't work>"
is not considered a bug, and we will not implement other features to make all
the SHOW commands usable inside a cursor just because someone exploited a bug.


mysql-test/r/sp-error.result:
  Only allow declaring cursors for SELECT statements to avoid
  possible further confusion/problems.
mysql-test/t/information_schema.test:
  Only SELECT statements are allowed in cursors.
mysql-test/t/sp-error.test:
  Add test case for Bug#29223. Non-SELECT statements in cursors now
  yields a parser error.
sql/sql_yacc.yy:
  Rework DECLARE CURSOR statement to not allow non-SELECT statements.