sql_yacc.yy and sql_parse.cc cleanup


sql/lex.h:
  INSERT ... UPDATE syntax
sql/sql_parse.cc:
  merged SQLCOM_REPLACE and SQLCOM_INSERT the same way their _SELECT counterparts were merged long time ago - just to remove duplicate code and be consistent :)
  now nobody needs SQLCOM_REPLACE* at all :))
sql/sql_yacc.yy:
  INSERT ... UPDATE syntax
  cleanup