Refactor and fix scuttle and scrap code

Refactor and fix scuttle and scrap code

Factor unit_drop_cargo() out of scra(), scuttle_ship(),
scuttle_land(), fix it up:

* Some messages were sent as bulletins instead of printing them.

* Nukes were always destroyed.  They're now treated exactly like other
  cargo.

* scuttle destroyed some cargo silently, and listed other cargo as
  "scuttled".  It now simply lets unit_update_cargo() running from
  carrier prewrite callbacks list all cargo "lost".

Simplify its callers.  scuttle_ship() and scuttle_land() are now
trivial, inline and remove.