unix, win: add netmask to uv_interface_address

Include the netmask when returning information about the OS network
interfaces.

This commit provides implementations for windows and those unix
platforms using getifaddrs().

AIX was not implemented because it requires the use of 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.