we change THD::system_thread from a 'bool' to a bitmap to be able to
distinguish between delayed-insert threads and slave threads.
- Fix for BUG#1701 "Update from multiple tables" (one line in sql_parse.cc,
plus a new test rpl_multi_update.test). That's just adding an initialization.


sql/repl_failsafe.cc:
  comment to warn about this unused code
sql/slave.cc:
  Now thd->system_thread is a bitmap, not a bool.
sql/sql_class.h:
  'bool' for THD::system_thread is not accurate enough; sometimes we need
  to distinguish between delayed-insert threads and slave threads;
  so changing THD::system_thread to a bitmap (uint).
sql/sql_insert.cc:
  thd.system_thread is now a bitmap
sql/sql_parse.cc:
  We need to initialize thd->lex.select_lex.options in mysql_init_query();
  it's already initialized in dispatch_command() but replication calls
  mysql_parse() directly, thus bypassing dispatch_command().
  Not initing it here leads to a query influencing the next query,
  in the slave SQL thread.
  The initialization in dispatch_command() must be kept as this
  command uses the variable in tests, even when the command was not a
  query (i.e. when mysql_init_query() was not called).