Unload prio and checkers libraries during shutdown.

Unload prio and checkers libraries during shutdown.

Our statup sequence is 'load_config', 'init_checkers', and 'init_prio'.
Both init_* functions reset the list of prio and checkers, which is
unfortunate as in load_config, depending on the multipat.conf, would
load prio and checker libraries. This results in double-loading of
the libraries and a memory leak.