gsocket: Fix g_socket_get_available_bytes() on Windows and OS X

gsocket: Fix g_socket_get_available_bytes() on Windows and OS X

On Windows and OS X, FIONREAD on a UDP socket gets the total number of
bytes available, not the number of bytes available in the next packet,
which is the more useful number (and how the function always behaved
on Linux).

On OS X, fix this by using SO_NREAD. On Windows, fix this by doing a
MSG_PEEK recv() into a giant buffer, since there is apparently no
other way to get the information.

https://bugzilla.gnome.org/show_bug.cgi?id=686786