Test for library reinitialization

Changing some plugin and reinitializing the library should (not) result in the
new code being used if plugins reload is (not) requested.
Clear errors instead of just freeing them

g_clear_error also sets the error to NULL which is often required by subsequent
GLib calls.
Make forcing specific plugins work

The bd_init and bd_reinit functions should take an array of plugin
specifications so that users of the library may force some specific plugins to
be used instead of the default ones.
Test for forcing specific plugin(s)