pegbox: Relicense under GPLv2.

The choice to use GPLv2 is for license compatibility with the Linux kernel;
 while no actual code has (yet) been copied therefrom (as pegbox is thus far
 entirely in assembler and there is no Z80 Linux port), Linux has been and
 continues to be a major source of design inspiration for pegbox.  Thus,
 although from a legal perspective pegbox may not be a derived work, I feel
 that acting as though it were is an appropriate courtesy.
pegbox: added list.s with doubly-linked-list primitives