Set the actual number of bytes written in the win32 SIOCGIFCONF ioctl handler

Testing Done:
Sent a file from windows to linux on both xmpp and bonjour. verified only 2 interfaces were found and that bytestreams were used instead of in band.

Bugs closed: PIDGIN-17123, PIDGIN-17293, PIDGIN-17516, PIDGIN-17704

Reviewed at https://reviews.imfreedom.org/r/2018/