Added SetDoDaylightCycle() and IsDaylightCycleEnabled() to cWorld.

I need this for a GameRule plugin.
Using a negative number for the stopped time is a protocol-specific detail. I'd prefer to have that wrapped inside the `cProtocol` descentant, and the SendTimeUpdate function should take the regular time of day and additionally a flag whether the day cycle is enabled; let the protocol decide how to send that to the client.