unify stack implementations

Graphviz contains a number of inline implementations of a stack data structure:

• initStk implementation duplicated in at least five places
• SFIO's sfstack
• cgraph's gstack_t
• cgraph's save_stack_t
• rbtree's stk_stack
• ortho's stack
• sparse's IntStack
• circogen's nstack_t
• cricogen's estack
• tred's estack
• sccmap's stack
• gxl2gv's ad hoc Gstack

I have not analyzed how much these overlap, but I hope we can all agree this is more stack implementations than we would like to maintain. As a first step, de-duplicating all the initStk copies would be progress. I have not yet investigated which is the best target library to merge them into.