We don't want such a mode to affect other uses of randomness, though. Thus use
a separate random number generator object for these two functions, and use a
fixed seed for it if the SC_RAND_REPEATABLE environment variable is set.

As RAND() is implemented in sc, and RANDBETWEEN() is implemented in scaddins,
it was a bit hard to figure out where to add the new functions needed, without
having to over-engineer things with UNO. (This functionality is totally
Calc-specific, but neither sc nor scaddins has any public (non-UNO) API.)
Caolan suggested the formula module, which seems like a good enough place to
me.

Change-Id: I4b0cb327392e51a18bce28478af91b0174d6b726