xorshift* PRNG

This needs testing to ensure I didn't mess anything up switching function names around.

Our PRNG sucks. This is probably obvious. I wish I had known better at the time I implemented it, but oh well.

The replacement is an xorshift* PRNG variant with period 2^32 - 1 (meaning that the PRNG state will loop after four billion calls ... that's not likely to happen), versus the old PRNG's period of about 2^22 (?). The output is also much more random and less predictable; the old PRNG would fall into a predictable loop of output after about 4000 numbers were generated, which isn't much.

The PRNG here also outputs numbers as fixed point from [0,1) (that's 0 to FRACUNIT-1, in other words) instead of single bytes at a time. This makes it much easier to calculate things for, say, P_RandomRange and P_RandomKey. A new macro, P_RandomChance(p), is now in use that returns true p percent of the time, where p is a fixed_t probability from 0 (0%) to FRACUNIT (100%).

This doesn't affect netgames at all; the code for seed saving and restoring is identical (aside from a check to prevent seed being set to 0, which breaks xorshift PRNGs). Demos break, but A: duh and B: they're already broken by all the changes to physics to accommodate slopes.

P_Random is deprecated in Lua, as the function was renamed to P_RandomByte. Aside from that, nothing special.