Fix argument to FIONBIO ioctl

Fix argument to FIONBIO ioctl

The FIONBIO ioctl takes as argument a pointer to an integer, which
should be either 0 or 1 to indicate whether nonblocking mode is to
be switched off or on.  The type of the pointed-to variable is "int".

However, the set_nonblocking routine in libstd/sys/unix/net.rs passes
a pointer to a libc::c_ulong variable.  This doesn't matter on all
32-bit platforms and on all litte-endian platforms, but it will
break on big-endian 64-bit platforms.

Found while porting Rust to s390x (a big-endian 64-bit platform).

Signed-off-by: Ulrich Weigand <ulrich.weigand@de.ibm.com>