1. ack, a.out, minix headers (moved to /usr/include.ack),
minix libc
2. gcc/clang, elf, netbsd headers (moved to /usr/include),
netbsd libc (moved to /usr/lib)
So this obsoletes the /usr/netbsd hierarchy.
No special invocation for netbsd libc necessary - it's always used
for gcc/clang.
Headers that will be shared between old includes and NetBSD-like includes
are moved into common/include tree. They are still copied in /usr/include
in 'make includes', so compilation and programs aren't be affected.