[libnetmap] Fix 32 bit compilation on gcc-6.3

[adrian]

Summary

This fixes 32 bit compilation (lib32 and native 32 bit target)
under gcc-6.3.

Test Plan

• compiled on gcc-6.4 for amd64 and mips targets

[imp]

You're going to need a lot more than 'fix compilation' for these patchs. Why do you need it? It super uglifies the code for no benefit.

[adrian]

• it fails on 32 bit compilations and we don't special case libnetmap to only compile on 64 bit platforms
• it fails on 64 bit platforms when compiling lib32, which blocks the amd64 buildworld
• since nr_options is a uint64_t and we're using it as a pointer type, we really should not be assuming pointer == sizeof(int type) like this

[imp]

I think that it would be better to use the nmreq_foreeach_option macro in these locations. It looks like it would expand to the same thing and future proof things a little better...

[adrian]

Update with imp's comments