When we access IPv6-related functions, we load the corresponding system
library using the `LoadLibrary()` function, which is not the recommended
way to load system libraries.
In practice, it does not make a difference: the `ws2_32.dll` library
containing the IPv6 functions is already loaded into memory, so
LoadLibrary() simply reuses the already-loaded library.
Still, recommended way is recommended way, so let's use that instead.
While at it, also adjust the code in contrib/ that loads system libraries.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
6f745cc4f1