UDF can be created from any library in any part of the server
LD_LIBRARY_PATH.

Allow to load udfs only from plugin_dir.
On windows, refuse to open udf in case it's path contains a slash.

No good test case for this bug because of imperfect error message
that includes error code and error string when it fails to dlopen a
library.


mysql-test/mysql-test-run.pl:
  Since plugins are allowed to be open only from plugin_dir:
  - there is no sence to update LD_LIBRARY_PATH
  - there is no sence to add plugin_dir arg by default
  - set UDF_EXAMPLE_LIB_OPT and EXAMPLE_PLUGIN_OPT to be used by
    udf and plugin tests accordingly.
mysql-test/r/plugin.result:
  Updated test result (we report addition warning).
sql/sql_udf.cc:
  Allow to load udfs only from plugin_dir.
  On windows, refuse to open udf in case it's path contains a slash.
mysql-test/t/plugin-master.opt:
  New BitKeeper file ``mysql-test/t/plugin-master.opt''
mysql-test/t/udf-master.opt:
  New BitKeeper file ``mysql-test/t/udf-master.opt''