plugin_dir option backported from 5.1

mysql-test/r/udf.result:
  result fix
sql/mysql_priv.h:
  opt_plugin_dir and opt_plugin_dir_ptr declared.
sql/mysqld.cc:
  'plugin_dir' option added
sql/set_var.cc:
  'plugin_dir' option added.
sql/sql_udf.cc:
  opt_plugin_dir added to the udf->dl path. Warn if it's not specified.
sql/unireg.h:
  PLUGINDIR defined.