unix, win:  add netmask to uv_interface_address

Include the netmask when returning information about the OS network
interfaces.  The end goal is to get this back to node's `os.networkInterfaces()`
function.

This commit provides implementations for windows and those unix platforms
using getifaddrs().

AIX was not implemented because it requires the use ioctls and I do not
have an AIX development/test environment.  The windows code was developed
using mingw on winxp as I do not have access to visual studio.

Tested on darwin (ipv4/ipv6) and winxp (ipv4 only).  Needs testing on
newer windows using ipv6 and other unix platforms.