Since this calls I_WaitVBL, which resets the frame time, it was essentially just like calling a real-time timer anyway and nothing in it required a specific 0-timepoint.
The same applies to the ZScript interface. All it needs is a millisecond-precise timer with no semantics attached.
- moved timer definitions into their own header/source files. d_main is not the right place for this.
- removed some leftover cruft from the old timer code.
