lib: child_process spawn handle ENOENT correctly

child_process spawn wasn't handlig ENOENT correctly on Windows.
This is half a fix for test-child-process-cwd.js.
The other half is going into libuv.
build: add libicu i18n support

Adds a --with-icu-path= switch to the configure script.  Requires that
the user checks out the copy of libicu that's bundled with chromium to
a fixed directory.  It's still a little rough around the edges but it
works.

Fixes #6371.