Reduce compiler/libraries/headers cases to only two

Reduce compiler/libraries/headers cases to only two

	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.