PLUGINS WITH LIBMYSQL_PLUGINS

Not loading pre-loading plugins was caused by the fact that
during pre-loading process there is no access to plugin-dir
value. So the only path available for searching pre-loading
plugins was default value (PLUGINDIR macro).

Fixed by introducing LIBMYSQL_PLUGIN_DIR environment
variable which can contain user-defined path to plugins.
mysql_load_plugin_v function modified to use
LIBMSQL_PLUGIN_DIR value if it exists and if plugin-dir
option value is not available.

Thus if LIBMYSQL_PLUGIN_DIR does not exists then behaviour
does not change. If LIBMYSQL_PLUGIN_DIR exists then it
unconditionally affects pre-loading plugins and it affects
other (loaded on connect) plugins only if --plugin-dir
option is not specified.

After the patch --plugin-dir option still has no effect on
pre-loading plugins. But user can specify plugin search path
with LIBMSQL_PLUGIN_DIR environment variable.