INFORMATION_SCHEMA.SYSTEM_VARIABLES

1. @@boolean_var differs from SHOW VARIABLES
2. @@str_var ignored variable charset (which is wrong
   for path variables that use filesystem charset)
3. @@signed_int_var in the string context was printed
   as unsigned

it can never be set to NULL, so it should
not use MY_I_S_MAYBE_NULL flag either

old fix only generated a warning for the *first* row
in the output

replace flag SHOW_VALUE_IN_HELP (that's not really an
independendent flag, as it should only be used for
NO_CMD_LINE sysvars) with a special getopt.id type.

their only purpose was to distinguish between
SHOW and SELECT and there are cleaner ways of doing  it.

* ignore the OPTIMIZER_SWITCH_ENGINE_CONDITION_PUSHDOWN bit
* issue a deprecation warning on 'engine_condition_pushdown=on'
* remove unused remains of the old pre-5.5 engine_condition_pushdown variable

MDEV-6606 Server crashes in String::append on selecting sql_mode inside anonymous block

MDEV-6603 SBR failure upon executing a prepared statement with input placeholder under anonymous block

Normally, Prepared_statement object rewrites the query on execution
to replace ?-placeholders with values. The rewritten query may be written
to logs (including binlog) or stored in the query cache.

But for compound statements, the whole block is prepared and executed,
while contained statements are logged individually. So it doesn't make
sense to rewrite the original statement block. Instead, we need to rewrite
every contained statement. SP is already doing it to replace SP variables
with values. Let it rewrite PS parameters too in the same loop.

Fix query rewrites in PS code - it was memcpy-ing the same query tail
many times. Instead use the same logic as in SP code, copy query pieces
into the destination buffer.

Extract this logic into a separate class Rewritable_query_parameter
with Item_param and Item_splocal inheriting from it.

Create a helper class Copy_query_with_rewrite that incapsulates
the query rewriting logic, use it in SP and PS.

originally based on the patch by Antony T Curtis

originally based on the patch by Antony T Curtis

cleanup sql_yacc.yy: rename rules for loops with and without label to follow BEGIN...END rule naming

preserve the Item_param type of the param_marker in the parser,
don't upcast cast it to Item

* split my_var class in three - base my_var and two descencants, move
  properties into descendants, remove if(), create a virtual method instead
* factor out the common code in the select_var_ident parser rule

instead of copy-pasting its methods into a new class