gxl2gv: replace custom 'slist' string stack with generic stack

gxl2gv: replace custom 'slist' string stack with generic stack

Similar to previous changes to `gc` in 4e2875fd7376338259dcb3ccc8f029d58bdf22dd,
this replaces some duplicated functionality with the generic Graphviz stack
implementation. Apart from reducing code duplication and complexity, this
removes an unnecessary rounding of allocations. The prior code rounded up
`slist` allocations to `sizeof(void*)`, but `malloc` effectively does this
internally anyway. This also adds checks for allocation failures that were
previously missing.

Gitlab: #1793