(no actual SLOC changes)

(it's still technically "undefined" behavior anyway)

P_RandomChance is now a macro for something that should happen a
certain percentage of time.

P_SignedRandom was moved to a macro. Nobody cared.

# Conflicts:
#	src/p_inter.c

This actually passes most diehard tests, as opposed to the old RNG.
It's also similarly fast.

Internally the PRNG generates a fixed point number from [0,1) now,
which makes P_RandomKey and P_RandomRange much easier to
calculate. P_Random is just a simple shift, as well.

Also, the lack of floating point math in P_RandomKey and
P_RandomRange now is probably for the best.