Clear GModule pointer to prevent accidental re-use

This makes it harmless (as it should be) to call
chimara_glk_unload_plugin() more than once. Since it is called in any
case when running a new game, client code calling it externally would
occasionally result in problems.
Reset Glk style hints at beginning of program

The Glk program can change the style hints so that different styles
display differently. These style hints should not carry over into the
next execution of the Glk program, or even a different Glk program. This
resets them at the beginning of a Glk program execution.