Allow Specification of the Length of the Sparkle Animation

[Vivit-R]

sparkle used to be a boolean option whether or not to show the the sparkly magic resistance animation. This branch changes it to a compound option that specifies how many frames the animation should run for. The argument to the option is optional and the default value is 21, so there will be no change in the behavior of existing .nethackrcs.

I had opened a PR about this against 3.6 a few months ago, but it couldn't be done then because it required changing the flags, which would have broken save compatibility with the rest of 3.6. Now that 3.7 is being worked on, that won't be a problem.