[Note: this is an updated version of Marc's patch #1187.  I realized I hadn't pushed that, but when I went to test it, it didn't compile for ARM.  I ended up doing some more restructuring in the process of fixing that problem.]

syscall emulation: Clean up ioctl handling, and implement for x86.

Enable different whitelists for different OS/arch combinations,
since some use the generic Linux definitions only, and others
use definitions inherited from earlier Unix flavors on those
architectures.

Also update x86 function pointers so ioctl is no longer
unimplemented on that platform.

This patch is a revised version of Vince Weaver's earlier patch.

passes 'util/regress quick'