Review how RNG seeds are generated/used in multiplayer

In multiplayer, the RNG is seeded in `game_level_init()` in freespace.cpp using `Netgame.security`, in `shipfx_blow_up_model()` in shipfx.cpp using `obj->net_signature`, and potentially by SEXP in `rand_sexp()`.

The `security` value is generated in `multi_sg_init_gamenet()` in multiui.cpp. The range of possible values is 1-32766, presumably because `RAND_MAX` on Windows was and still is 32767.

However, once PR #3332 is merged, then the RNG can generate values as high as `util::Random::MAX_VALUE` which is `0x7fffffff` or 2,147,483,647.

We might want to take advantage of this increased range, to help allow for a wider range of multiplayer experiences.

Similarly, since the RNG is re-seeded in `shipfx_blow_up_model()` using an object's `net_signature`, which has a pretty limited range of possible values, we may want to explore ways to increase the range of possible seeds, for the same reason as above.