The idea of the patch is to separate statement processing logic,
such as parsing, validation of the parsed tree, execution and cleanup, 
from global query processing logic, such as logging, resetting
priorities of a thread, resetting stored procedure cache, resetting
thread count of errors and warnings.
This makes PREPARE and EXECUTE behave similarly to the rest of SQL
statements and allows their use in stored procedures.
This patch contains a change in behaviour:
until recently for each SQL prepared statement command, 2 queries
were written to the general log, e.g.
[Query]   prepare stmt from @stmt_text;
[Prepare] select * from t1 <-- contents of @stmt_text
The chagne was necessary to prevent [Prepare] commands from being written
to the general log when executing a stored procedure with Dynamic SQL.
We should consider whether the old behavior is preferrable and probably
restore it.
This patch refixes Bug#7115, Bug#10975 (partially), Bug#10605 (various bugs
in Dynamic SQL reported before it was disabled).


mysql-test/r/not_embedded_server.result:
  Since we don't want to log Dynamic SQL in stored procedures,
  now the general log gets only one log entry per SQL statement.
mysql-test/r/sp-error.result:
  - remove obsolete tests
  - a better error message for the case when a stored procedure that
  returns a result set is called from a function
mysql-test/r/trigger.result:
  - a better error message for the case when a stored procedure that
  returns a result set is called from a trigger
mysql-test/t/sp-error.test:
  - a better error message for the case when a stored procedure that
    returns a result set is called from a function.
  - move the comment to its place (end of file).
mysql-test/t/trigger.test:
  - a better error message for the case when a stored procedure that
  returns a result set is called from a trigger
sql/item_func.cc:
  - we need to pass sql_command explicitly to get_var_with_binlog, because
  when creating a query for SQL prepared statement thd->lex->sql_command
  points at SQLCOM_EXECUTE, which is not listed in the list of update
  queries.
sql/log_event.h:
  - remove an extra copy of the previous sentence
sql/mysql_priv.h:
  - fix declarations of sql_prepare.cc API
sql/share/errmsg.txt:
  - a new error message, when one attempts to execute a prepared statement
  which is currently being executed (this can happen only in Dynamic SQL
  at the moment).
sql/sp_head.cc:
  - extend sp_multi_results_command to return different flags for a
  command (and rename it)
  - add support for SQLCOM_PREPARE,SQLCOM_EXECUTE, SQLCOM_DEALLOCATE
    to sp_get_flags_for_command
  - replace multiple boolean sp_head members with uint m_flags
  - a fix for a crash when user variables are used in a stored procedure
    and binlog is on. A temporary fix for Bug#12637 "SP crashes the server 
   if it has update query with user var & binlog is enabled", which actually
   stands for stored functions: now instead of a crash we break
   replication if a user variable is used in a stored function which 
   is executed in prelocked mode.
sql/sp_head.h:
  - replace multiple boolean flags of sp_head with uint m_flags;
  - add flag CONTAINS_DYNAMIC_SQL
  - use this flag to error if a stored procedure with Dynamic SQL is
    called from a function or trigger.
sql/sql_class.cc:
  - Statement_map::insert should not delete a statement if it exists,
    now it's done externally to be able to handle the case when the
    statement being deleted is in use.
  - remove extra code (free_list is already reset in free_items)
sql/sql_lex.cc:
  - add lex->stmt_prepare_mode; we can't rely on thd->command any more,
    because we don't reset it any more (Dynamic SQL requirement is that
    PS are as little intrusive as possible).
sql/sql_lex.h:
  - declare bool LEX::stmt_prepare_mode
sql/sql_parse.cc:
  - move prepared statement code to sql_prepare.cc
  - change declarations (refactored code)
  - better error message when one attempts to use Dynamic SQL or a 
    stored procedure that returns a result set in a function or trigger.
sql/sql_prepare.cc:
  - major refactoring to ensure PREPARE/EXECUTE commands do not reset global THD
    state and allow their use in stored procedures.
  - add Prepared_statement::flags and use it to ensure no recursive execution
    of a prepared statement is possible
  - better comments
sql/sql_yacc.yy:
  - enable PREPARE/EXECUTE/DEALLOCATE in stored procedures
  - produce an error message on attempt to use PREPARE/EXECUTE/DEALLOCATE
    in a stored function or trigger
mysql-test/r/sp-dynamic.result:
  - sp-dynamic.test results
mysql-test/t/sp-dynamic.test:
  - a new test for PREPARE/EXECUTE/DEALLOCATE in stored procedures.