[cmds] Add accurate train timing to sl

[ghaerr]

Adds an accurate 40ms "cycle time" between each train re-display, so that the sl train should always chug along at a constant speed on all systems (or at least as fast as possible on slower systems). sl now calculates a minimum cycle of 40 ms and will add a delay if the last display cycle was quicker than that. This now helps the illusion that all trains have equal conductors (yes, I'm talking about your 8088 systems @Vutshi).

In order to continue to use sl as an indicator of how fast or slow your system may actually be without a delay, a new -f option was added: sl -f will run the trains off the tracks if your system is fast enough (which is the same as sl previously with no option).

To summarize our steam powered evolution:

sl     # train running at 40ms/frame or slower on all systems
sl -s  # this train emits more ANSI sequences than slow systems can handle
sl -f  # this train doesn't delay or stop for anybody

This finalizes all of the reported problems in #1619.

[Vutshi]

At this stage, ELKS might just happen to possess the most advanced railway system on the planet.