spank: consolidate globals in plugstack.c

The use of globals in plugstack.c is cumbersome and prevents the
future expansion of spank plugins, e.g. calling spank plugins from
multiple contexts within the same process or reinitializing the
spank plugin state.

This patch consolidates the current globals (spank_stack, spank_ctx,
spank_optval, and option_cache) into a global "spank stack" structure
and expands many of the functions internal to plugstack.c to operate
on a struct spank_stack instead of globally.