2. All have_xxx variables are now selectable.


sql/set_var.cc:
  1. sys_have_xxx variables added.
  2. Removed sys_var *sys_variables[] array as we don't need it any more.
  3. void set_var_init() changed to use sys_var_xxx chain insted of sys_variables[] array.
sql/set_var.h:
  1. add_sys_var() method added to the sys_var class. It's called from constructors
  to chain all successor objects. The first one is stored in the 'static sys_var *first'.
  The total number of variables is in the 'static uint sys_vars'. Each sys_var successor object
  has the 'sys_var *next' pointer to the next one in the chain. 
  2. sys_var_have_variable class introduced to make all have_xxx variables selectable.
sql/sql_lex.cc:
  trg_new_row_fake_var(0, 0) replaced with '(sys_var*) 0x01' as we don't want to have such fake variables in the sys_var_xxx chain.
sql/sql_lex.h:
  Proper use of the changed trg_new_row_fake_var.
sql/sql_yacc.yy:
  Proper use of the changed trg_new_row_fake_var.