WIP tools mkhybrid (mkisofs) for mac68k/macppc install ISOs

[Izumi Tsutsui <tsutsui%ceres.dti.ne.jp@localhost>]

> > As one of the QEMU Mac machine maintainers I've been doing some testing
> > against NetBSD 8.0 and NetBSD current, and I am unable to boot
> > NetBSD/macppc current ISO images under qemu-system-ppc due to OpenBIOS
> > no longer being able to read any files on it:
> 
> There is a long standing issue with our build system that requires manual
> internvention to create working HFS hybrid ISOs - so the regular daily builds
> do not get this treatement and only formal releases have actually bootable
> ISOs.

Honestly it's unlikely for us to get HFS ready makefs(8) so
I've tried to port OpenBSD's mkhybrid 1.12b5.1 to NetBSD's src/tools.

 https://github.com/tsutsui/netbsd-tools-mkhybrid

One major difficulty is that mkisofs (and cdrtools) uses its own
complicated build system using custom configure etc., so currently
the above WIP files use fixed config.h for NetBSD host and also use
src/tools/Makefile.host with libnbcompat.a. Then it won't be build
on non-NetBSD hosts.

It might be better to use cdrtools-2.01 sources but it looks
a bit more complicated (like libscg and libschily stuff etc).

As noted in the README.md, there are several issue on pulling
mkisofs options required for distrib/cdrom/Makefile from newer
mkisofs/cdrtools versions. Currently mkhybrid gets SIGSEGV on
calling free(3) on writing images so I put a kludge to disable
the free().

I'm just tired to debug it, so I'll punt it to github..

---
Izumi Tsutsui