Make non-.exe externals work again 7ebac8cb94f3a06d3fbdde469414a1443ca45510 made launching of .exe externals work when installed in Unicode paths. But it broke launching of non-.exe externals, no matter where they were installed. We now correctly maintain the UTF-8 and UTF-16 paths in tandem in lookup_prog. This fixes t5526, among others. Signed-off-by: Adam Roben <adam@roben.org>