Optimize fixed length MyISAM rows to use pread/pwrite.


BUILD/compile-pentium-debug-max:
  Also build embedded server
libmysqld/lib_vio.c:
  Add vio_poll_read()
myisam/mi_statrec.c:
  Use pread()/pwrite() instead of seek+read/write
mysql-test/r/multi_update.result:
  Fix multi-table-delete test
mysql-test/t/multi_update.test:
  Fix multi-table-delete test
sql/filesort.cc:
  Fix multi-table-delete
sql/mysql_priv.h:
  Fix multi-table-delete
sql/sql_class.h:
  Fix multi-table-delete
sql/sql_delete.cc:
  Fix multi-table-delete
sql/sql_parse.cc:
  Fix multi-table-delete
sql/sql_select.cc:
  Fix multi-table-delete
sql/sql_table.cc:
  cleanup
sql/sql_unions.cc:
  cleanup
sql/sql_yacc.yy:
  cleanup/ optimize
sql/structs.h:
  Fix multi-table-delete
sql/uniques.cc:
  Fix multi-table-delete