I don't like that idea because the zone index handling is already awfully long. I defined a new macro `UV_PLATFORM_HAS_IP6_LINK_LOCAL_ADDRESS` and protected the helper function with `#ifdef` instead.

Perhaps we could move `uv__parse_zone_index` to platform-specific files (e.g. [include/uv-private/uv-win.h](https://github.com/joyent/libuv/blob/master/include/uv-private/uv-win.h)) in order to keep [src/uv-common.c](https://github.com/joyent/libuv/blob/master/src/uv-common.c) cleaner, possibly defining the helper function as a macro instead?

What do you think?