with plugin-load-add that are already registered at mysql.plugin
- issue just one error message, without this extra warning
- don't abuse ER_UDF_EXISTS, instead add a proper error message for plugins
- report started initialization for each plugin source
fails when running with ps-protocol).
The problem was that when running in --ps-protocol mode mysqltest.cc
didn't close created prepared statements. So, the plugins could not be
unistalled because there was a prepared statement using them.
A fix is to add a dummy statement that forces mysqltest.cc to close
the last prepared statement (which uses a plugin-defined table).
