Problem: net/if.h not properly included

In general, including system files in class sources is messy and
error prone. It is better to include these in czmq_prelude.h once
for the entire project.

However in czmq_prelude.h we don't have platform.h; this header is
also used by applications.

Solution: in classes, include platform.h BEFORE czmq.h. Then, do
all non-portable includes in czmq_prelude.h according to platform.h
macros.

Fixes #425, I think.