Windows: download busybox-win32 during make win-extras

copy it to echo.exe and printf.exe during make dist
This allows test/spawn.jl to succeed on Windows, since echo and printf
are only included as shell scripts in msysgit (and an echo.bat in
contrib/windows), but libuv cannot spawn shell or bat scripts.
distinguish between libraries and bundled exe's
Merge pull request #7446 from tkelman/busybox

Windows: download busybox-win32 during make win-extras