nonblock: fix setting non-blocking mode for Amiga

IoctlSocket() apparently wants a pointer to a long, passed as a char *
in its third parameter. This bug was introduced already back in commit
c5fdeef4 from October 1 2001!

Bug: http://curl.haxx.se/mail/lib-2015-11/0088.html
Reported-by: Norbert Kett