Modernize SGB code

[ascherer]

• RAII
• Use type bool
• ~~gb_flip§12: do...while without braces~~; no
• End @<Named sections@> with @;
• string literals are const char*
• Use snprintf instead of sprintf
• ~~gb_graph: Forward-declare typedef structs~~; no
• ~~miles_span§24 and §30: init_heap and init_F_heap should use parameter long d (0L)~~; done
• ~~Clean up #defines in gb_{games,gates,miles}.h~~; done
• Use size_t instead of siz_t (gb_basic, gb_graph, gb_plane, miles_span) (Caveat gb_graph§34!)
• gb_save§20 should be listed under system dependencies; Don says no
• …