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.

Makefile

@@ -98,6 +98,7 @@ clean:
 	$(MAKE) -C test clean
 
 cleandepend:
+	$(MAKE) -C lib cleandepend_all
 	$(MAKE) -C boot cleandepend
 	$(MAKE) -C commands cleandepend
 	$(MAKE) -C bin cleandepend