Added support for UPDATE.
Some minor fixes.


mysql-test/t/rpl_row_tabledefs_2myisam.test:
  Rename: mysql-test/t/rpl_row_tabledefs.test -> mysql-test/t/rpl_row_tabledefs_2myisam.test
mysql-test/extra/rpl_tests/rpl_row_tabledefs.test:
  Extending test to ensure that there is one more null byte on slave than
  it is on the master.
  Some cleanup.
sql/field.cc:
  Added support to find the last null byte for a field.
sql/field.h:
  Added support to find the last null byte for a field.
sql/log_event.cc:
  unpack_row() will now deduce the number of null bytes on the slave
  and use that when copying the null bytes from the row.
  Factored out code to copy "extra" record fields into separate function.
  Used that function to copy the "extra" fields when updating a row as well.
mysql-test/r/rpl_row_tabledefs_2myisam.result:
  Result change
mysql-test/r/rpl_row_tabledefs_3innodb.result:
  New BitKeeper file ``mysql-test/r/rpl_row_tabledefs_3innodb.result''
mysql-test/r/rpl_row_tabledefs_7ndb.result:
  New BitKeeper file ``mysql-test/r/rpl_row_tabledefs_7ndb.result''
mysql-test/t/rpl_row_tabledefs_3innodb.test:
  New BitKeeper file ``mysql-test/t/rpl_row_tabledefs_3innodb.test''