Add hook in dynamically loaded modules for unloading actions.

Add hook in dynamically loaded modules for unloading actions.

Dynamically loaded modules can't use reload cleanups, because they
may be unloaded by pruxdld's reload cleanup before their reload
cleanups get to run, causing DISK-RESTORE to attempt to execute
unmapped code.  Instead, pruxdld will now call dload_finalize_file
if it is defined in the file.

Use this mechanism in prx11.  Fixes DISK-RESTORE after launching
and exiting Edwin.  (Does not fix DISK-RESTORE of an image that was
saved while Edwin was running.  That is much harder.)