Fix plugin loading

Fix plugin loading

Plugins are built as shared libraries and are therefore installed into
$libdir/bind.  Meanwhile, the build system sets the NAMED_PLUGINDIR
preprocessor variable to $datadir/bind instead.  This prevents loading
plugins specified in the configuration file using just the shared
library name (i.e. without using an absolute path or a relative path).
Fix by setting NAMED_PLUGINDIR to the path that plugins are actually
installed into.