Fix mingw-gcc 'ordered comparison of pointer with integer zero' warning
Use custom windows spawning again; add various pref for system() spawn

Fixes #943. system() has issues with quoting of spaces, but may still
be needed due to bugs in the custom spawning code. system() will be used
if win_spawn_mode=1, otherwise leave it at 0.
Make default win spawn g_spawn_sync to avoid hang with > 4kb output